I want to paint over the chrome on the back of some lights.....what should I coat it with first before the paint? Just undercoat? Something more?

you have to take the bits to the electro platers n get the chrome stripped off.

the paint wont stick unless you do this first.

I have painted the backs of a couple of chromed indicator housings with POR after sanding the chrome as well as I could to roughen it up. Although I haven't used them since, the paint did go on and stay on. It would probably work even better if they were sandblasted.

POR makes a product called Glisten PC which is supposedly designed to go over polished or chrome plated surfaces. I haven't tried it yet, but I have just bought some so I'll be trying it out in the next couple of weeks.

Painting the backs of chromed things like indicator housings is a good idea as chrome is not a particularly good corrosion inhibitor by itself. The painted indicator housings I have are generally in much better condition than the chromed ones.