converting from 3 bulb to 2 bulb tails?

Here's one way to wire the new turn/brake light setup, I bench tested it so I know it will work as shown. The diagram only shows one rear light configuration, so you will have to duplicate this setup for the other rear light as well.

When the brake only is applied, it goes thru the left relay and to the bulb. When the turn signal is applied it activates the left relay, in case the brakes are applied, if the brakes are not applied, the turn signal power flows thru the right relay and flashes normally. If the turn signal and brakes are applied, the brake signal overrides the right relay and the turn signal feed interrupts or flashes the brake light signal when the turn signal flasher flashes. The diodes are needed to keep the power from backfeeding power to the individual circuits when necessary. You can get the 1N4001 diodes from Radio Shack(Part #276-1101).

One thing to be aware of is that a turn signal flasher(typically a 552) will only activate or flash when it sees a load on the other side, for example, the turn signal light bulb. This setup is wired so that if the brakes are applied, the turn signal bulb is isolated when the right relay is activated, so the load to this bulb is not present any longer. However, on most turn signal circuits, the flasher should be sending power to both the front and rear turn signal bulbs, so the flasher would still sense the load to the front bulb and the left relay should activate and the rear bulb should flash as indicated. If for some reason the front and rear turn signal feeds are seperate and the rear turn signal is not sensing a load causing the flasher not to flash, I believe you could connect a resistor to terminal 86 on the left relay, to simulate the bulb load, thereby flashing the brake signal.

Maybe auex or John DeRosa can come up with a simpler diagram.

If all you're trying to do is run a double bulb tailight assy in a vehicle that had a triple bulb setup you can get adapter boxes to do this.. the actual purpose of the box is to allow someone to run a trailer.

https://shop.easternmarine.com/index.cfm?fuseaction=catalog.prodInfo&productid=4993&categoryID=209

Jim

Yup.. one box controls both rear lights. I've used them in the exact same situation you're facing. Chrysler that was having different year taillights installed; the box makes it easy.

Jim