That looks simple enough for me
The DPDT switch here
http://www.jaycar.com.au/productView.asp?ID=SK0967 looks ok.
For the 12V relays
http://www.jaycar.com.au/productView.asp?ID=SY4066 SPDT
For layout I was thinking of hotgluing the relays to a piece of PC board with a diagram on the board and soldering to the pins .
re the paint on your 'stang, have you seen the latest Mazdas with a toffee apply paint job? The paint looks to be the same thickness as the toffee.