I'm one of those blessed souls that has never had a lick of trouble with stock buss bar/internal relay, lot of years, lot of miles.
I recognize that people do have problems and Vince's solution is significantly more elegant than a ford starter solenoid and a bunch
of ugly wiring with a pushbutton. It might have something to do with my replacing glowplugs, as a set, just about yearly as a matter of maintenance.
Untill they start to be $20 a pop I'll probobly keep doing that. I also haven't lived anywhere colder than central Oklahoma for a winter.
YMM, as they say, V.
Moving the relay out of the under-dash has been a "one of these days" projects I have yet to get to.
I did relay my reverse lights though, they fail more often in my world.

My other GP related project has been to retrofit a 240D Mercedes late style GP relay, it
is a single small unit with relay, fuse, separate GP wires, and I believe the later one's incorporated
afterglow and a temperature dependent feature. I do know that they do some match referencing
current draw on #1 and comparing it to the others to see if all is well, it nixes the GP light if
it isn't. Debatable usefulness, but it's interesting engineering for the era.
The unit and it's plugs are pretty attractive too in an industrial sorta way, it could still use
larger wires for voltage drop, but there are a lot more OM616/7's still on the road than rabbit diesels.
Been my experience that everyone with every diesel complains about there glowplug system.
Ever.
http://www.ecklersmbzparts.com/mercedes-240d-glow-plug-relay.html