I used to have a basic Blitz timer on my 5 turbo. It needed 2 earths bolted to the chassic, a wire to the handbrake switch to act as a cut out, a constant live feed then two more which joined onto the 2 ignition wires so it could monitor the electric pulses when revving. It showed it all in the instruction manual that came with it.
On my 5 I only had a simply immobilisor that cut in after the engine stopped so it didn't affect it, not sure how a your alarm/ immobilisor would handle it.
Have you turbo'd your 172 then, as there's no need for a timer on a NA car? it's to keep oil/ water moving through the turbo to cool it down before the engine turns off