My set up includes p1900 with the Mac-Pac cable; Mac mini installed in dash. This setup has been working perfectly but access to the power button on rear of the mini is a pain. Can I splice in additional- external power button?

For my first attempt, I found a small tact switch and wired it in parallel. This switch will power on the mini and also put it in sleep mode but now the mini wont power on nor goto sleep with the ignition. After turning off the ignition mini just turns off after the delay time.

What can be done to fix this and keep both power buttons?
THX