Tom -
1. Using a test light, connect to the B+ (positive) feed side of the horn and find a good ground. If the test light lights up, you probably have a horn ground problem. On the '28 the horn B+ terminal should be hot all the time.
2. Disconnect one head lamp socket, turn on the head lamps and using the test light again, see if the test light lights up while probing the feed line to the headlamp and using a good ground. If test light works, you probably have a poor ground through the head lamps.
3. If the test lamp doesn't light up on either of the first two checks, go to the back of the headlamp switch. Using your test light, prob the horn terminal, turn on the headlamps and prob the headlamp terminal. If the test lamp fails to light up on either or both, you probably have a bad headlamp switch.
When you installed the new wiring harness, did you connect the various wires to the correct terminals as marked on the back of the headlamp switch? For fun, ground the horn with a second wire to a good known ground and rotate the headlamp switch through all switch positions. If the horn blows while doing this maneuver, you probably have some mis-installed wires on the headlamp switch. Otherwise, you need to look into the headlamp switch operation.
Dick
