Figure out which wires are hi and low behind your drivers side headlight with a meter or test light.
Tap (tee) into each wire with a new wire, a Scotch-Loc connector works great.
Connect the hi wire to the trigger side (pin 85 on most relays).
Connect the low to switched side of the relay (pin 87).
Put 12+ on the other switched side of the relay (pin 87A). Use a inline 20 amp fuse and grab the power from the driver's side battery terminal.
Ground the other trigger terminal (pin 86).
Relay can be attached to the front side of the battery box.
You're done, beauty of this modification is if anything goes wrong if reverts back to the normal set up.
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Bill
'95 2500 4wd auto
'95 3500 5 speed heavy hauler
Not stock