Are there no regulations for side marker lights other than you must have something back there and it must be red for the rear?
That is not the case. Rear side marker lights and reflectors, like all vehicle exterior lighting devices, are technically specified in great detail. I'm not sure what you have in mind when you ask this question, but it looks as if you might be wondering if there are regulations stipulating that any given vehicle must have the side markers built into the rear combination lamps vs. separate on the side of the car body. The answer is no. Vehicles made after January 1, 1968 must have front amber and rear red side marker lights or retroreflectors, and vehicles made after January 1, 1970 must have front amber and rear red side marker lights and retroreflectors, and they must meet the design, construction, performance, and mounting requirements, but there is no regulatory preference for separate vs. combined installation.
hydro00, it's not clear to me what you're trying to get across regarding side marker lights. I linked him to lights and reflectors that will work, and as long as they're mounted in accordance with the applicable provisions of FMVSS #108, which simply require the rear side marker lights and reflectors to be mounted as close as practicable to the rear of the vehicle, I don't really see what more there is to it than that. However, since you've been banned for awhile on account of your persistent and willful disregard for the rules of this forum (not to mention your base rudeness) I suppose we will have to wait awhile for you to try and explain.