I'd consult Cynon, Ceej, and F1V1 on the headlights and tallights. They make great headlight and taillight assemblies for the cars in TMMC, ARLA, etc. Vinny can probably give you some assistance on it, but I can't guarantee that.
Doing headlights and tailights in GIMP is a bit doozy. It's not like Photoshop or Paint Shop Pro. It's essentially too basic to do headlights and taillights with GIMP because there aren't enough tools to do that with. In doing them with Photoshop or Paint Shop Pro, you'd have a much decent shot at making some.