Is there any way I can find/buy a wiring harness for a 99 and put into my 00? If not, what else can I do?

 

You can buy an adapter that changes 2 wire to 3 wire, they are made for trailer hook ups and not expensive

1999 has 3 wire tail lights, 1 park, 1 brake, 1 turn
2000 has 2 wire tail lights, 1 park 1 brake/turn

Google: CURT 56196

Just an example there are other makers of these adapters
They are made to be mounted under rear of a vehicle

You can hit a wrecking yard to get the rear 3 wire harness from a 1999 and older Ranger/Mazda with bulb holders, so you don't have to run extra wires
Put the 2 to 3 wire adapter where the 2000 harness meets the "1999" rear harness, the wiring is very straight forward, just weather proof the connections

But the actual 3 down to 2 wire conversion was done in the cab wiring, in the multi-function switch, so full conversion would be much harder and no reason for it

There are 2 more wires, Ground, of course, and the Back up lights

If you have a trailer light plug in you can wire that to either side of the adapter, OR have 2 trailer connectors, labelled, lol, for either type of trailer wiring

 

Were does the 99 taillight harnesses connect at? Is there a specific type of connector I need?

 

Drivers side frame rail at the rear, all Rangers have the START of the tail light harness at that location as far as I have ever seen, but haven't seen every Ranger, lol, or every year

Yes, I would think there would be different connectors when Ford changed the wiring from 3 to 2 wire brake/turn