Ok, found the black/white wire. It connects to a long wire that disappears into a harness that runs under the engine, near the lower radiator hose. Looks like it goes to the starter.

Confirmed that those two connectors are for the horns!

Still can't find the yellow wire...maybe for the coolant temp sensor? (that's two wires though). There aren't any other two-wire weatherpacks that I can think of.

2000 Mitsubishi Montero Endeavor, "Katsumi", 138k
1993 Toyota Tercel, backup vehicle, 93k
1989 Isuzu Trooper, "Beniko", R.I.P. at 233k

My 'Genuine Isuzu Parts' decals - for sale