I have a friend who I am trying to help out who has asked me to make an OEM momentary switch for his car turn on and off the power to an amp and nav unit. I have found a circuit to build that works with fans that looks like it disables the ground on the device to turn it on and off. I don't think this is a good idea for amp and nav unit. I know nothing about relays, but was wondering if it is possible to plug a relay in the place of the fan that will toggle the actual + power on and off to the devices. Again, I know nothing about relays, sso I might be asking to plug a square into a circle. Any other suggestions would be greatly appreciated. Below is the circuuit I was talking about.

http://www.cpemma.co.uk/flipflop.html