Had issue with only high speed working and only vents

Do you have a computer-controlled heater system, or are your mode doors actuated with common-sense switches and push buttons?

I am using a wiring diagram for the manual system. The light green wire is fan speed two between the switch and the resistor assembly. If there were to be a break in that wire, the fan should not run at all on speed two, however, the drawings are not always correct when it comes to showing the internal switching. If your switch has an "off" position, does that work? Some systems only have speed one as the lowest setting. In that case, that circuit is always on, then the other speeds bypass speed one. A break in any other circuit would make the fan run in the lowest speed.

Unplug the connector from the resistor assembly, then measure the resistance on each wire to ground. You should find close to zero ohms on only the black/tan wire in the "high" position. The other wires should read open circuit. You should find zero ohms to ground on only the light blue wire in the second highest position. Zero ohms on only the light green in the third highest position, and zero ohms only on the tan wire on the slowest position.

Measure the resistance between the light green and the tan wires.