Mitsubishi lights operate off switching from lo/high with negative switching. Most other vehicles operate with a positive switched set up. You need an adapter harness that converts from negative switching to positive switching.
It's about $25 for a nice tidy harness adapter.

89 SWB, V-6 5SPD, OME Shocks, 16.5X37's, 2"BL, SR axle, Gen2 brakes, Lincoln w/Superwinch