If you want brake lights without a mercury switch, you'r going to need some sophisticated piece of electronics to sit between the throttle/brake channel and the throttle/brake servo. It's not simple because you have to read the pulses comming off the control line and act appropriately. So it's not a simple sodering job.

I swear that I've seen light kits where it was a brake-light controller and a failsafe all in one.

I think I found it with a quick google search:

http://www.hyperdynelabs.com/rc_lights.htm

I searched for RC Brake lights if you want to try yourself.

-RobH