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Suggest , if you don't already have one, try to get a copy of the owner's manual for your 97 van. Looks good.
It has specs for all bulbs used and a diagram of the fuse/relay panel and what they protect.
Just type in ,Ford Owners Manual and you can obtain one from there.
A little side note,,,,
Keep a close eye on your 97 ford front brake pads. The only way you know they require replacement , if you don't check them regularly, is when you feel the metal to metal grinding noise of the pad metal destroying the rotors. They have no kind of notice like some have squealers when renewal is required.
Simple routine checks can save you some nice cash.
Diagram above locate circuit 14 (BR) wire that's the parking light, including front left + right, rear left + right and license lamp. If you don't have rear parking lamps but you have license lamp and front parking lamps working then you don't need to check on the 20A fuse, that means there is a break somewhere, perhaps at S306. Since you never mentioned your license lamp working or not, also check C301F/C301M if your license lamp is also not working.
Diagram below is for the brakes and you said they are working fine, so this diagram is for reference only, you can see circuit 5, 9 and 511. You can see circuit 57 (BK wire) are the ground for both, and when your brake lamps are working, you don't need to check the grounding.
Running board lights, never heard of that as factory light. What to do to turn it on? Parking light + that particular door or any door (just light dome light)? And all running board lamps light up at the same time?
Factory or aftermarket doesn't matter, do you have keyless entry? How about power door locks?