Not all vehicles are multi-livery capable.
For the 'gt17_sign_1' name thing, if you use CodeWalker to export the '.yft's to XML & then search them for 'template' & change 'template' to 'gt17_sign_1', the vehicle will use 'gt17_sign_1' texture instead of 'template' one.
Doubt that will make any difference to multi-livery support tho (it's essentially just a name change), as it's highly unlikely the mod author gave it multi-livery support & then didn't bother to implement/advertise it. More likely, the car only supports one livery at a time.
You'd have to contact the mod author for confirmation of all that, but you most likely only have two possible solutions:Ask the mod author to give it multi-livery support If the car is unlocked & can be edited in Zmodeler3 - Edit it & give it multi-livery support yourself/pay someone to do it etc.