You are not blowing fuses because more than one power feed is grounded. Most system are HOT to ground! This isn't! With multiple grounds in the system and the other car shut-off for repairs you now have something bypassing the doors. Some of the older systems had a coil at the beginning of the doorlock ckt and at the end? If the grounds were on contacts in that circuit....no fuse would blow and you just jumped them out! More than likely when the other unit was shut-off that provided the "perfect-situation". I do agree these car's had to be doing some strange things when the system slowed down! That's why we did maintenance back in the day and performed system checks on weekends and at nights....This would of come up an been addressed! Oh....That's why you keep 79 with 00 and 511 with 512. Most guy's over the years use whatever they can find....Those colored wires are there for a reason!