Originally Posted by theelcaminoman
This graph looks good... now explain it lol. Im not much with graphs
With no power going to the first relay (one on the left) 87a is connected to pin 30 allowing current to pass through. So when the brakes are applied current from the brake signal goes to pin 87a and out pin 30 to pin 86 of the other relay (right). This activates the relay (coil inside energizes, producing magnetic field, pulls switching element (thing with arrow) to pin 87. So now pin 87 is connected to pin 30, which allows current to pass through the relay, coming from the battery (through the fuse, squiggly line), out the relay and to your lights which are grounded.
For the parking lamps part or turn signals (I think yours are wired using just one wire). When the parking lamps are on, current flows to pin 86 of the first relay (left) and energizes the coil, causing pin 30 to be connected to pin 87. Now the park lamps act like the brake signal to the second relay.
Originally Posted by OTRtech
Be a lot simpler and cheaper just to use diodes .
Take two 1N4004 diodes (rated at 1 amp-should be ok for a marker lamp) and twist the cathode ends together.Connect the twisted end to the lamp.
On the free ends of the diodes,connect one to the park lamp wire,connect the other diode to the brake.
This may be a better way, it really depends on the wattage of the bulbs he is using. I just like using relays because I don't want to have too much current flowing through stock wires. That and he will have around 1 volt less output at the markers than the rest of the lights due to the voltage drop of the diodes.
Dual filament bulbs would probably be a good idea like Scrambler said.