They used to make "latching" relays .

I'd think all you need is a heavy duty continuous duty relay for the 1.5MM wire, not the positive cable to the starter .

Maybe an inverter would work .

Supposedly automotive electronics work until below 10VDC but that's from my factory training in the 1980's.....

Most breakerless ignitions work down to just above 9VDC .

? Have you done the multimeter & fuse pulling test yet ? .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better