Actually, no wiring diagram should be needed! What you've got is a failed blower motor resistor block, the method used to slow the blower motor to the lower speeds that you're now missing. What happens is a diode in the assembly blows and power stops flowing through any but the high-speed (direct-wired) circuit.
Cause for the failure can be an older fan motor that is drawing a bit more power than it used to. Or it might just be a bad resistor block... tough call. My advice is that if the motor runs quietly, put a $15 resistor block in first and see how long it lasts. They can last for years like this. Or they can burn out in a matter of days/ weeks at which time a blower motor is certainly recommended. Your call.
You'll find the resistor block above the right side under-dash silencer pad. Remove the pad, look up and you'll see the black plastic resistor block held in place by two 8mm sheet metal screws. OK... you might need a mirror to actually see 'em! The 5-wire connector will draw your attention to it if you're having problems locating it.
It's plug-and-play, remove the old one, install the new one. The only tool you'll need will be an 8mm socket, short extension and ratchet, all in 1/4" drive.
Write back if you have any questions. Be glad to help.