89 F150, oops, wrong tank!

Got that low-pressure pump in the tank, high-pressure pump on the rail thing. The big pump only sucks gas from the rear, although when the front tank’s selected, only the front in-tank pump goes active. What’s up, and where do I start in finding and fixing? Is it that funny can with the six lines (two to each tank and one to the motor?) apologies in advance, but if I reply, it’ll be in a new thread, as my phone won’t make replies.