Hello

I'm trying to help a friend install an after market head unit into a 1993 Ranger. There was not one when he purchased the truck. We bought the appropriate wiring harness, etc. There is 1 longer rectangle plug in the truck and 1 square plug with a single, thick blue cable. I can get the unit to power on with the longer rectangle plug but the square plug provided with the harness that is for the speakers does not match up. Keep in mind this harness can be used with numerous different vehicles and I do not know what the stock square plug was intended for. What I'm wondering is if there needs to be an adapter or if there is suppose to be a 2nd rectangle plug for the speakers? The truck is pretty beat up so it wouldn't surprise me if the PO cut that plug, etc. It doesn't appear so because there are no bare wires in the dash that would make it obvious.

The picture provided is not the exact brand of harness he purchased, but the gray/black plugs have the same cords. Gray has the power options, black the speakers.

tl;dr Should there be a black adapter in the truck as well to get the speakers to work, or is there an adapter for the square plug with blue cable?

Thank you for any advice, recommendations, etc