Here's the headlight switch test procedure from my Ford software:
Illuminated Entry, Aerostar.
NOTE:
Before performing diagnosis, verify illuminated entry system function. The illuminated entry module will not turn off the interior lamp if it is turned on by the headlamp rheostat, or if the door is open.
Quick Checks
1. Verify that circuit to interior lamp is working. If not, check fuse.
2. With the ignition switch in ACC or RUN position, verify that other systems which share the same accessory fuse are functional. If not, check fuse.
3. If any of the above systems are not working properly, service the inoperative circuit and then recheck the operation of the illuminated entry system before proceeding with the following complete check-out procedure.
Pretest Checks
1. Terminal numbers referred to in the following procedure relate to the wiring connector attached to the illuminated entry actuator assembly. The terminals are numbered from left to right (when the connector is held as shown).
2. Any malfunctions diagnosed in Step 4 of the test procedure could be located in either front door.
3. To trace and/or service various circuits, refer to the respective vehicle wiring diagrams.
4. Check system fuses (two) before proceeding.
Test Procedure
1. Connect a 12-volt test light between Terminal No. 8 and a good ground. Test light should glow at all times with ignition switch in RUN or OFF position. If not, trace circuit 54 (light green with yellow stripe) back to fuse panel and service.
2. Connect a 12-volt test light between Terminal No. 7 and a good ground. The test light should glow when the ignition switch in ACC or RUN position and go out when the ignition switch is turned to OFF or LOCK position. If not, trace Circuit 296 (white-purple) back to fuse panel and service.
3. Connect a jumper wire between Terminals No. 6 and 8. With vehicle doors closed, interior lamp should be on. If not, trace Circuit 53 (black with light blue stripe) and service.
4. NOTE: When operating door handle, hold in raised position.
Connect an ohmmeter between Terminal No. 2 and a good ground. The ohmmeter should show over 10,000 ohms (open). Then, operate each outside door handle in turn; ohmmeter should show not more than 50 ohms (closed). If readings are not as specified, then trace Circuit 465 (white with light blue stripe) back to latch switches and service.
5. Connect a 12-volt test light between Terminal No. 1 and Terminal No. 8. The test light should glow. If not, trace Circuit 57 (black) back to ground and service.
After Testing
If all tests check out OK and the illuminated entry system is still malfunctioning, the actuator assembly is the cause of the problem and must be replaced.
