dotPeek is a .NET decompiler which can decompile code back to semi-recognizable code. It's a good idea to use a decimal->hex converter to get back the natives they call.
For .asi scripts and obfuscated .NET scripts you're gonna have to do some more work but maybe other people know what's to be done.
Is it still possible to mod Vehicles/Maps/Peds in SP?
Yes. They went and reversed the C&D against the OpenIV team.
Also I would like to know if there is a thing like "mod packs". I tried modding GTA 4 back then when the first nice vehicle mods were released but I remember how frustrating it can be to mod every single car in the game.
Read up on this quick start guide. I think add-ons and .oiv installers for big packs suit your needs.
Last but not least: Is there something like a launcher so I can decide whether I want to play with or without mods?
Read the quick start thing I wrote. Basically just use the mods folder method and then you can disable or enable all mods by just changing one file.
Walk us through the steps you do and link the vehicles you use. You're probably just forgetting something.
If you used the mods folder method you can restore the files with the original files. If you haven't, you can let Steam/RGSC Launcher verify the game files and undo your mods, so you can start over fresh with a mods folder.
As you might know my Manual Transmission script has some issues with the Logitech G920 + ScriptHookVDotNet, and probably it affects some other wheels too.
I've made a build with some more logging built in. Please test this special build and report back with Gears.log.
Instructions:
Gears.asi and Gears.pdb to the GTA folder. Replace the old Gears.asi.ManualTransmission/Gears.log to pastebin.com. It contains info that could help me fix the problems.I'd particularly appreciate some reports from Logitech G920 users, as that seems to crash for some weird reason when using ScriptHookVDotNet.
Thank you 