On the mtrbd. USB connection follows the USB standard, starting at pin 1. If what you're asking is what wire harness setup is, then that's entirely unique even though it follows once properly connected the same results. If you have actual separate wires per pin, then I can easily see your situation. You need to google for USB wiring and there several examples out there. As the other poster offered a meter would help to determine basic wiring. Also, since you have an old 1500 model, that maybe at a period when USB connections did vary, pin wise amount. true USB std. now is 10-pin, minus 1 pin for "key purpose" to ordinate pre-made wire harness. -OR- as use the pin 1 on wire harness to pin 1 on mtrbd.(noted). Yet, most wire harness use 4-wires(when separate), the idea behind 10-pins at mtrbd. is to properly set it all up, thus a hub for 2-USB ports If you're dealing with a 4-wire connection, then find the Grd, 5V+, and Signal. Confusing, you bet in older and early USB setups, thus the 10-pin connection became the norm. Colors do vary, sorry they do, so while a clue they're not always followed. If you're lucky, the wire harness has stickers/labels ID'ing what they do. Maybe, your USB labels are [+], [-], [data or signal], [grd], or [5v]. Google for wiring schematics.
If this becomes too painful, then use a USB PCI type card to add USB ports for quick use of the PC for the time being, cost is low.
I recently put the guts from an E machines 1500 in another case because of mechanical switch problem with the case front. I can not find usb pin out ID for the usb Hub on the am35 motherboard. Would like ot install a card reader /usb internally but everything I can find has different plug wiring including the blocked connection. For what it is worth the wire colors shown in the manual do not match what is in the block. Any help or reference to a site with the answer would help>