I think I'm getting closer to resolving this issue! At least I've figured out that bypassing the bulkhead fixes the issue!.... I'm about to check under the dash like you said!
Your issue is somewhere between the two ends of the bypass wire Likely a connection. and IMO 14.9 is still too high. if not previously mentioned the Ammeter terminal nuts are notorious for coming loose. DO Disconnect the battery while checking them
Likely a connection. and IMO 14.9 is still too high. 
