I just did this. I have a dual battery set-up and my 88 import has two relays. The ECU should be behind the glove box (mine is RHD). I don't know what a drop resistor is, or where it would be located. I believe from reading here and Pocuck that drop resistors may have been used on some models with 6V glow plugs. At any rate, I first checked by disconnecting the forward relay from it's control wire and hot-wiring it for 10 seconds. Worked good. So...
I purchased a momentary switch on ebay (toggle for me). I disconnected the control wire to the forward relay and ran wires from the positive terminal on the aft battery and the forward relay across the firewall, then through on the right hand side next to where the wiring bundle goes through. From there to my switch - next to a unused space to the left of my steering column. I put a rubber grommet in the firewall hole. Works fine.

I just had my head replaced and the glow plugs were replaced as well. The mechanic used Bosch. From what I have read, it's better to get MMC plugs that were specified for your particular set up - I guess there are lots. I think my ecu works okay, but is not compatible with the Bosch plugs. Too expensive to change. I think this will work better any way, and should extend the life of the plugs. Trial and error will determine the time to use the momentary switch. After mine has started once, it's good to go the rest of the day without glow plugs unless it sits for many hours.

What I have now is momentary switch on for a bit, instant start, clickety-click from the aft relay for 10-15 seconds (apparently what it's supposed to do) and I'm on my way. No more for the rest of the day - of course, it never gets really cold here.