rewiring reverse lights?

Yes that was the one I was asking about.  It would be very simple if you could change the sockets to accept a dual element lamp.  If that is not an option, we can still make it work with the sockets you have.  On the socket for the reverse lamp, are both the ground and hot wires accesible?  You will have to cut both and using 2 relays, will have to switch between series and parallel wiring.  When park lamps are on, the lamps are going to be wired in series.  Thus making the bulbs burn at half brightness.  When the brakes are depressed, they will be wired in parallel.  Full brightness.  There is a diagram on this site for switching speakers from series to parallel.  I will look at it and see if it needs any modifications to do this.

This is the diagram for the speakers.  It will take some modifications to make it do what you are looking for.  I have no problem modifying it if you tnink you are going to do it.  Let me know if I need to modify it.

We may need a third relay if you want this to work during the day.  When I drew the diagram, I was thinking you had daytime parking lamps.  A different post here. The diagram below will work at night.  I will need to modify it for brake lights in the daytime.