I had the same problem with my auto, ('90 mpfi ea82), figured that all the relays & switches had burnt contacts as voltage dropped very low (7 volts) when cranking/starting, (way too much trouble to find & replace them all), so I did as the previous posts have suggested & bought a 30 amp horn/driving light relay, (with an attached mounting bracket), a metre & a half of 3mm wire, (of at least 40 amp capacity), an inline 30 amp fuse & necessary connectors, then mounted the relay near the solenoid using a brake line mounting bracket & 10 mm bolt on the firewall.
The wiring diagram posted above is the only way to do it.
This permanent fix uses the car's original wiring to only run the low current draw of the new relay (0.5 amp), which it can easily do, instead of actually powering the heavy current needs of the solenoid (25 amp), which are supplied by the new relay, wire & fuse, so don't use old worn out crap for this.
