what's a 12v src only when engine is on?

Hey guys, I'm looking for a wire that outputs 12V+ only when the engine is running (not just ignition on), any ideas? Possibly the alternator output wire or the fuel pump power? The easier to find, the better. Thanks!

A vehicle make and model may make it easier for someone to help you.

Charge lamp is a good start, pretty standard on vehicles. (would be your best choice I think) Another option may be oil pressure switch if your vehicle is equipped w/ one. Fuel pump is tricky because you get a fuel pump prime when you key on regardless of starting. So that could become a nuisance for you under certain circumstances.

Use a dual coil latching relay from the starter side, then use the ignition wire so that when you turn off the ignition, this goes to neg and flips the coils and turns things off. Omron, available from mouser, farnell etc.

It's a 2003 Mustang GT. I was thinking about the latching relay, as it seems to be the simplest option.

I was going to say avoid oil pressure, and fuel pump power - especially of both are directly related! Carby vehicles use the alternator charge signal (charge lamp) else a spark/points etc sensor. EFI vehicles can also use charge lamps & spark sensors, and air-flap switches, but usually some ECU signal - maybe injectors (akin to spark). But depending on what you want to do, avoid oil pressure. When used for fuel pumps or fuel cut-offs, oil pressure mechanisms fail most (competition) safety regulations. And when used as engine preservation systems by cutting the fuel, they certainly fail - even with EFI.

PM me with an email address, I'll send you the part number and a diagram.