Questions for electronics gurus

[Jay Whitely]

Hey guys,

I have a set of two electric fans in an S14 Silvia
right now.  Each fan has its own relay.  Signaling the
fans to come on is a single wire coming from a coolant
temperature switch. When the engine coolant reaches
190 degrees, the switch completes a ground and the fan
relays are activated.

I am looking to *also* activate the fans with a single
wire that sees 12V when the A/C compressor is turned
on.  I know I will have to wire in a relay to invert
the signal, but here is my question:

Should I add a diode to the wire from the A/C
compressor signal wire to isolate the two circuits
from each other, so that I do not get "backflow" of
electricity?  If so, should the arrow of the diode
point toward the relays or toward the A/C compressor?
Laslt, what size or value diode should I use?

Thanks a lot,

=====
Jay Whitely
Tampa, FL
1996 200SX SE-R
GTi-R powered, F-MAX Stage 2