i have a 77 300d with the glowplug relay(or pre-glowplug relay) under the dash. when i try and start the car it buzzes and i dont get any power to anything else in the car. ill tap it and it will stop buzzing and then the dash lights and radio will come on. but when i turn the key to the starting position nothing happens. if this relay is bad would it keep the engine from turning over? and is there a way to hot wire the car? that way i could take it to someone that would have a better idea on how how to fix it.

 

The obvious mission information here is the condition of the Battery and what type of Voltage drop the battery is experiencing when you turn the Key on.

I have not read a good write up on trouble shooting the Older type of Glow Plug setups.
They are expensive but you could just try replacing the Relay.

The first thing I would try is pulling out the Relay Completely and try starting (In you description you did not say if the Engine would crank or not you said it did nothing.)
and see what happens. If everthing electrical is working OK after pulling out the Glow Plug Relay could be a reason to suspect the Relay itself is bad.
While it is out; and if you can pry off the Aluminum Cover and have a look inside to see if the Contacts are burned up or melted.

I would also check your Glow Plug Strip Fuse which on some models is under a plastic cover on the Fire Wall (US models).

I do not know how well your Glow Plugs would hold up if you ran a temp wire to the Strip Fuse to heat the Plugs so that you can get started.

Another option would be to install the updated Pencile Type Glow Plug kit that comes with a Relay (not the one under the dash but one you would install under the hood).

There is also a possibility that you have several electrical problems going on at the same time and the Relay is not working correctly due to that.

two items connected with starting are the Ignition Switch itself wich of course activates the relay. The other is the Neutral Safety Switch (automatic trans) which also goes through the starting circuit and is connected to the Ignition Switch.

After checking the other stuff I mention and that does not clear things up my wild guess is that there is a possibility of a worn out Ignition Switch.

 

Good lord I was looking and there are 2 types of Glow Plug Relays for your year and model depending on you chassie number.
And the one in the Metal Box is around $165

If the Relay turns out to be no good; many opt to get the update kit as it cost about as much as the Relay alone and the Pencil type Glow Plugs work better and make starting easier.
Below are some links concerning installing the kit.