Dodge Diesel/Cummins Mechanic Advice Needed

Wasn't sure what section to post this in, so Mods feel free to move if I put it in the wrong place. Here's my issue:

99 Ram 2500 w/5.9 Cummins. Was having a slow to start issue along with every so often while I was driving down the road the Air Bag light would come on. Well, slow to start turned into no start. Lift pump was changed 8 years ago, injector pump three years ago, put on a used injector pump recently that was supposedly good and just replaced the lift pump again after a local shop recommended it, but they actually recommended buying an aftermarket Fass or Air Dog pump instead of a new Cummins pump.

The lift pump is a new Cummins pump; I know they're junk, but I'm just trying to get the truck running to sell it and don't want to drop big money into it. I know I shouldn't have wasted money on a used injector pump, but once again, just trying to get it running to sell.

I'm getting fuel to the injector pump from the lift pump. I'm also getting a small amount of fuel out of the lines at 1, 3, and 4 injectors. I"m getting the same amount of fuel that I was before I changed out the lift and injector pumps.

I read in my manual in the wiring section and noticed the Air Bag system power is intermingled with the fuel system power. Any chance the two problems could be related?

I'm getting fed up and could really use some advice, besides spending big money on an after market lift pump and new injector pump. I'm really leaning towards new injector pump, but I can't stand to spend the money, especially if it could be something else.

Thank you in advance.

Brad