Russ et al,

I just recently had a similar experience, so I have been thinking about it.

1) Your circuit won't work.  It's OK for turn signals or brake lights,
   but not for turn signals and brake lights.  If the stop lights are on,
   the turn signals won't be seen.

2) 30 years ago when I was asked to design some auto tail light stuff,
   I came across a government regulation that precluded using diodes
   and maybe even transistors in the tail light current path since a
   maximum allowable voltage drop was identified.

3) So, the other day, I made a design attempt using relays because of
   my recollection of #2 above.  I came up with two 2 relay circuits
   that do not pass lighting current through any diodes.  I have not
   tested them though:

   a) 2 double pole double throw relays
      2 1 amp diodes (to make hazard lights work)

      This circuit has the flaw that the brake light opposite the flashing
      brake light will have a slight flicker.

   b) 2 single throw double pole relays
      2 1 amp diodes
      2 large electrolytic capacitors

      This circuit has the flaw of being a bit more expensive than the
      previous circuit.  The capacitors likely cost a lot more than
      the savings from simpler relays.  The values of the capacitors
      are determined by the coil characteristics relays chosen.

I'm sure that other folks have better designs than mine.  Let's hear from you.

Thanks,
Mike