Some people have asked about the cost of tooling on these things. I have over 20 years experience in die casting (same as injection molding only we use metal instead of plastic) and can help answer that ...
Because of the temperature of the material and the pressure used to shove the material into the mold (we are talking hundreds of tons) you can't use just any material here. The material you have to use to make tail lights is not like the cheap junk plastic used in things like the dinasour making units you see at fun parks. Also, you have a lot more detail (fine lines, screw posts, etc) than the cheap things can handle. Additionally, these days, you have to conform to Department of Transportation regulations for reflectiveness, brightness, etc, that you did not have to meet when our cars were new. All this requires a lot of almost microscopic detail when cutting tooling from hardened steel and testing (with the appropriate documentation) to show the parts meet spec.
Also, you are actually talking 4 molds; two molds for each side (LH and RH). In the case of the tail light, you probably would be talking the base and the outside. The two halves have to be constructed so they consistently fit EXACTLY perfect (or very, very close to it) with various types of overflow passages that are used to pull the molding material (in this case plastic) through past the farthest points away from where the material comes into the mold. The molds have to also be designed to fit specific types of machines (the equipment that will run it) so the various fittings match up and properly work.
I suggest that anyone interested contact a tool and die shop in their area and ask about prices to see for yourself. Also, remember that you probably won't sell any more than 100-120 sets and there are other expenses connected with running these things.