My 1984 300D has a busbar mounted on the passenger side fenderwell, covered with a plastic lid. the trigger wire for the soleniod is the white one going down past the battery. Just clip a remote switch lead there and clip the other one to that fat bundle of wires in the next terminal over, and you're in business. FWIW the purple wire comes from the keyswitch.

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Nate Stanley
(Currently Benzless)
1985 F-250 6.9l 170K
2009 SCION XB 36.5K
2003 LS430 78K
2012 Kubota B 2320