If I'm reading this correct, you have a push button start. And want another start method in the form of a key under your dash in case the button fails?
If so, you should already be using a trigger start onto a relay using the push button start. Or maybe your not and that's why you want a high current key.
If it's using a relay already, just run the key in parallel with the push button start. If not, I havn't a clue where you would get a high current key switch.