Access to solenoid wire, (210) 2002 E320s4

[Paul T]

Short story: I want to wire an emergency start switch into the line that fires the starter solenoid. There is no access from underneath the car, with this area being filled with front drive shaft and heat shield, down-pipes, etc.
Would you know where I might find / tap into the wire that eventually makes its way to the solenoid activation terminal?

Longer story; 2002 e wagon w/ 125k miles, sometimes does not start. intermittent problem, not battery related. Happens with either key. Car will start (so far) two hours later. All the dash lights come on, but no starter activation, no crank. Been to the shop; no codes found. Poss bad ignition switch, starter, other?? I would like to be able to apply power to the starter when this next happens to rule out the starter. It looks like ab**** to get at.
Thanks,
Paul T