there is a small black wire that runs down to the starter solenoid, if u splice into it and run 12v to it the starter should turn.
if u really didnt want to spend $ on a new starting switch , wiring a small door bell type of switch to the starter will be a cheaper way, i did that to one 85 190e reason the alarm unit was dead it also splice the starting too, when unit dont work u cannot get car started.
So I ran s separate door bell switch, later on I discovered the alarm unit and cprrected the wiring, I can use the key to start car properly.

Only issue with own starting switch u will have no transmission lock out when not in Neutral or park, basically start anytime too can be more dangerous in the hands of weak brains!
or atleast locate a power source only when u turn on power, otherwise the switch has no power.

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