I am trying to wire in a set off traffic lights as my brake lights/turn signals on a rat rod truck. My question is how would I proceed with it? IE, they are a 120v ac design led bulb and would need them to run on a 12v DC setup.
Something like this would preserve the traffic lights as much as possible.
A few comments.
The inverter can be the cheapest, crappiest you can find as long as it's reliable. You aren't driving a computer here.
The relay should be a 12-volt coil with contacts rated for 120 volts at several amps. You simply connect the coil in place of the brake light bulb.
It's entirely possible that the LED modules (one per indicator) are not actually capable of running at 120 AC. At the least, it's probably rectified in the traffic light controller box. In this case, you would need to put a full-wave rectifier somewhere between the inverter and the LEDs. It's even possible that the LED modules, in addition to being DC, are not 120 volts. But you need to determine this yourself.
As Grant suggested, stick with 12V bulbs. LEDs are neat, but it might be easier to work with regular car brake/turn signal/fog lamps, which already run off 12V.
Next, try to find one that fits the existing socket, and if that fails, find sockets for the bulbs you like that will be relatively easy to retrofit into the housing.
The main thing is to put a bulb that's easy to deal with (i.e. one that uses 12V DC) inside the housing and illuminating the lens. After that, follow the same procedures that apply to all a car's lights.
Low current switch -> relay -> lamp or
high current switch -> lamp.