I did search but confused the hell out of myself. FSM didn't help either.
First, what is the optimal on off temps for a 280. Is it on 185 off 170 or on at 200 off at 185?
Second, what is a good ignition on power source for the relay? I am going battery to 30A fuse to relay to fan positive. relay ground to temp switch. Fan ground to chassis. I just need good ignition on power source hopefully in the engine compartment.

Are you using an electric fan as a primary cooling fan? The normal thermostat is 185. Where is your sensor located?

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Chattanooga, TN
82 white w/ red n/a coupe 120,000 miles.
83 2+2 314,395 miles at college AGAIN
96 Pathfinder, 96 Maxima
90 300ZX 2+2, 82 ZXcoupe, 81 200sx, 72 Camaro SS, 68 Mustang, 1950 Chev PU fully restored
Pics:[www.zcar.com]

electric fan will be primary. temp switch is in upper radiator hose.

You could tap into the power source of your fuel pump relay. I think it is under the pass seat, but I'd have to look it up for sure.

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Chattanooga, TN
82 white w/ red n/a coupe 120,000 miles.
83 2+2 314,395 miles at college AGAIN
96 Pathfinder, 96 Maxima
90 300ZX 2+2, 82 ZXcoupe, 81 200sx, 72 Camaro SS, 68 Mustang, 1950 Chev PU fully restored
Pics:[www.zcar.com]

using a thermal switch to trigger the power relay is probably the best way to do it. Use an offshoot from the primary power to the thermal switch to the power relay and it is done. No thinking involved as long as the thermal switch is properly located. why mess around with finding something from the ignition. If the car is not running no way it will get hot enough to trigger the fan.