Let me first say that I am no expert into technicalities of the underdash wiring of P1800's, having owned one only since May this year (-72 ES).(if it works, don't fix it!).

However, reading the instruction manual lately I figured out that there is a separate on-off dimmer switch under the dash, to the very left and similar to the swith for the map reading light on the opposite side. This switch' only function is to dim the long beam light and the overdrive light for long drives at night. It has nothing to do with the rheostate switch which controls all the other dash lights.

Could it be that this circuit creates the problem described? If I activate this switch on my car, the long beam light will almost disappear whereas the overdrive green light will dim significantly. With switch in off position, my long beam blue indicator is very "loud & clear", even during daylight.

Just wondering.

Ola
-58 544
-72 P1800ES
-97 850