This might help you troubleshoot your problem
https://www.kidde-fenwal.com/Media/Data Sheets/series-35-66_24-VAC-hot-surface-ignition-control-F-35-66.pdf The blower motor is powered/started by the Fenwal. If you look at the wiring diagrams there are some discrepancies in terminal labeling. TH on your Fenwal is the same as W in the listed wiring diagram. 24VAC on your Fenwal is the same as R in the listed wiring diagram. The fenwal is powered to "standby" by having 24V between GND and 24VAC. The ignition sequence is started by having 24V between GND and TH. When the ignition sequence is started the Fenwal first checks to make sure that the air pressure switch is "open" and not stuck in the closed position without the blower running. If the air pressure switch is open, it will then power the blower motor by closing a relay for one leg of the blower motor through the NO (normally open) contacts at F1/F2. Do you have 24V between GND and 24VAC at the Fenwal? Do you have 24V between GND and TH when you are asking the heater to start? If your ground is good, you could try jumping the "hot" (ungrounded) leg of the 24V from the transformer to TH on the Fenwal to see if the ignition sequence will start. Will try to answer any questions you may have if you feel comfortable/safe troubleshooting the electrical with a multimeter.