a cheap servo ... that...came out of a RC toy car (HPI Minizilla
Depending on how much the toy originally cost, I wouldn't necessarily expect it to work like a hobbyist servo. Depending on production volumes, it could have a completely proprietary control protocol.
I'd try to examine the toy using an oscilloscope to see what controls it expected.
Try finding an R/C model shop, and buy a cheap Futaba 3003.
Make sure you read the servo library documentation carefully.
Make sure you join the grounds between your separate power supplies.
I soldered and wired up everything perfectly
How do you know this?
Again, an oscilloscope is great help.