Movement sensor

[mister_e]

Mister_e,

I also thought to use the + wire from the ignition key lock but
it would not be so easy to get it and it's not always located
at the same place.

I hate that suggestion but you could still tap your wire to the fuse box. Professional installer will tap their wire directly to the ignition switch. It's always at the same place.. but of course never the same color depending of the car Brand. Not as this hard....

I thought it should be something like a mercury switch or a
metal ball etc.
Let's say if you push the car it should be already enough to switch
on.

Yeah those mercury switch could work, but if a van pass close to the car, it may trig a false event if not properly installed. Another thing... if the car is parked in a not flat area (few degree) the sensitivity will be affected as well.

In the past we used that kind of switch in car alarm, i debugged a lot of faulty installation and also i remove tons of them to replace with usual shock sensor... which don't apply in your specific case.

I will prefer a ignition switch + maybe a VSS signal combination... yeah the Vss signal is sometime a pain in the LALALA to find as most car cluster haven't that signal directly at the cluster connector but computer mixed signal...kind of data line.

OBD ic?... i don't think it worth the price if you just need a VSS signal.

Easier to read a Tachometer signal... if you got a tach signal... at least the car is started. This said, you just need to detect a few HZ frequency. You can take the signal from the Car ignition coil, or injector. For Diesel, you might need to read the voltage at the alternator output OR create your own tach signal with a Hall-Effect sensor...

Nothing is perfect