I've an intermitent problem with my glowplug circuit which is currently resolved. I had to start the truck by lifting the bonnet and manually operate the glowplug relay. Approx. 5 seconds holding this with a few second delay before turning the key and wallop the engine started with far more enthusiasm than standard. I usually preheat 3-4 times before turnng the key and the engine starts with a bit of chugging.

My question is: If I were to wire in a switch to bypass the usual circuit am I likely to cause any damage to anything?

The only thing I can think of is that the longer you're glowing the shorter the life of the plugs as they're only designed to be used for a couple of seconds.

I vaguely recall that the driver's door switch was supposed to activate the plugs, so they were doing their thing before you'd got your bum on the seat. (Not a bad idea really)

I would venture that the longer you ran them in one go, the more chance you have of damage. Maybe a 5 second timer on a momentary switch?

The automatic circuit is supposed to shut them off when the combustion chamber reaches a certain temperature (yes really; I've seen the plug light stay on for times that vary with ambiant temperature). The only downside to a manual switch is if you use a latching switch and don't shut them off you can burn them out. So I'd suggest using a hold to make switch. Any competent motor factor, accessory shop, or auto electrician will understand what you want from that.

the great thing is with using a momentary action switch in this application is that you wouldnt even need to fiddle with any live wires, you all you need to do is earth the glow plug relay, so you only need to run one single wire from the switch to the fusebox, then just ground the other side of the switch to a suitable earth point near the switch

there are a few other interesting options with regards to the realy itself too, you can get self timed relays which latch on then switch off automatically after 5 seconds, also ive seen automotive style glow plug relays which are for marine applications which will only latch on once until the power is interuppted then it resets itself... they are a little bit more complicated to wire up but its all possible