@ImWinning try using this
https://www.gta5-mods.com/tools/xmlspreadsheetv

Thanks to Microsoft Excel, Windows Commandline, Eddlm for making the AutoTurbo script that I have included with permission, sp4c3m4n for his LSC color index, and Notepad++

Here's how to utilize the spreadsheet. I will take you through step by step the process of adding one car to the spreadsheet all the way from the .zip you download to moving your car pack files into the dlc.rpf in your dlcpacks folder.
Step 0: Head to the Constants sheet and name your car pack in the second cell (Cell B2)
https://i.imgur.com/G7lCkHK.gifv
Step 1: Acquire a car mod from this website and extract it
https://i.imgur.com/TytRBO7.gifv
Step 2: Using OpenIV extract the .ytd and .yft files from the dlc.rpf (if AddOn style) OR if a replace option is available use those files. Copy the .yft/.ytd files into the "Input" folder.
https://i.imgur.com/kc65O1E.gifv
Step 3: Once done adding all the car models run the .bat file in the "Input" Folder and open up the spreadsheet. Head to the SizeData sheet and paste it on the second line (Cell A2)
https://i.imgur.com/o5yKYk0.gifv
https://i.imgur.com/z307jjp.gifv
Step 4: Head to the main sheet, the "Cars" sheet and fill in the name of the files you moved under "Model ID" and start filling in information about the car.
https://i.imgur.com/uIHJ0Z5.gifv
Step 5: Once you have the Make and Model filled in, hit calculate in the bottom left (Or F9, Or the button under the formulas tab), and then you can use the first button to search wikipedia to allow you to fill in the rest of the information. If wikipedia does not have 0-60 or top speed information there are two additional buttons to help find that information as well.
https://i.imgur.com/hfuxR9p.gifv
Step 6: The "Low" column changes the layout of the car from standard to low (enter Yes if the characters head clips the roof in game). The color section allows for up to 8 Color schemes to be added per car. For only primary car colors just enter the code corresponding to the color in the table in the top left. For primary and secondary use a "/" inbetween. (for example 88/147). Use the color search to find out what colors the car comes in.
Step 7: Add in the tuning parts to the tuning sheet. (Will expand tutorial when I get a chance)
Step 8: Hit Calculate and check the name of the first pack in Pre-Output (Your prechosen name+1,2,3,etc.). Create the folder structure for part one of your pack. Copy the Template folder and rename it to the name in Pre-Output. Open all of the files you see me opening here (with notepad++)
https://i.imgur.com/u6d5BQA.gifv
NOTE: for step 9 you need to have a macro set up (and preferrably bound to a key) that replaces """" with (nothing) and "" with " in notepad++. You need to run this macro after anytime you paste data from excel into notepad++ (except for global.oxt)
Step 9: Set up excel and notepad++ side by side and head to the Output tab in excel and do as I do:
https://i.imgur.com/m3hjnO1.gifv
Step 10: Save all of the files open in notepad++ and close it. Open up OpenIV and head to mods->update->x64->dlcpacks. Create a new folder with the same name as before and make a rpf file and name it dlc. Drag all of the files in your car pack folder over except for global.oxt
https://i.imgur.com/a7EkaGC.gifv
Step 11: Open up americandlc.rpf in OpenIV and drag your global.oxt in. Open up tuning_mods.rpf and move your tuning files in. Open up vehicles.rpf and drag your car files in.
https://i.imgur.com/EYnZcfc.gifv
Step 12: edit the dlclist.xml and add that same folder name as an entry
https://i.imgur.com/HRBqk64.gifv
Step 13: If any of your cars have turbos ("Yes" in the turbo column) head to the sheet "AutoTurboOutput" and click the top cell and copy it over to the AutoTurbo .ini
https://i.imgur.com/JWQSnx0.gifv

Thats it! Hopefully this will explain enough of how the spreadsheet works so that people can utilize it. I will add another part for tuning but It is pretty straight forward if you look at it.
https://i.imgur.com/0RMwbdT.jpg

there may be something I forgot and it may not work for you or something might be slightly off
BEAR WITH ME PLEASE
excel is giving me lots of trouble I had to move the spreadsheet over to another spreadsheet entirely so some cell values could possibly be off I haven't done extensive testing.
Also I want to revamp the lookup tables for how it decides which audiohash to use.
Some other features are that it creates a modkit for each car depending on how fast it is. the slower cars get a bigger engine upgrade to allow them to be realistically fast. all the cars also have lower options for the suspension kits!

NOTE: I haven't tuned the handling for the cars with the turbos, so the cars that autospawn with the turbos will have a faster 0-60 time than you input

I will update when I get a chance but I feel like at this stage it is functional enough for people to use.

@ImWinning under your content.xml you have 69chargermods.rpf listed under the <dataFiles> section but not in your AUTOGEN

try this
https://pastebin.com/J4Z7MGzT

don't have time to actually test the dlc.rpf in game to see if that fixed it but I will later when I get a chance if that doesn't fix it.

@ImWinning show me the file layout for your dlc pack, your content.xml, and your tuning mods .rpf contents

Paste a screenshot of your materials browser and textures browser

What exactly is your problem? You replaced the carbon texture with a black texture and it's not showing up?

Are you exporting it as the car model it's ""replacing""? The .yft isn't 0KBs is it?

You've got part of a <Kits> section lodged in a wheel section.

Also if your combining them just use notepad++ and merge all the <Item> entries for <Kits>,<Lights>, etc. should be super easy

here's the correct version (with the <Item> Kit put in the <Kits> section instead of wheels)
see if this works:
https://pastebin.com/Cs6yQnjQ

Post your (pastebin) .meta files and I'll fix them

I'm currently working on a spreadsheet that makes merging DLC extremely easy but I am not sure when it will be finished and don't want to release it until it is complete.

@Moff_Man491 What zeroxzerotwo said.

If you find you can't tune anything in LSC except for lights, plate then just add this to the <Item> entry (or change the existing tag section to this) in carvariations.meta in the dlc.rpf:

  <kits>
    <Item>0_default_modkit</Item>
  </kits>