I have a 1990 sw20 and the immobiliser on the dash decided not to work (for about 5 minutes) the other day, is there a way of bypassing it and just installing a switch or button?

It will be possible but you are going to have to take a closer look at all the wiring.  You'll need to identify where the car loom is split by the immobiliser and then either remove it completely and join the wiring up or just effectively short it out.
 
So you need either a wiring diagram or you just have to get stuck in and identity all the wiring going to it, shouldn't be hard at all though.

I have a wiring diagram, it's just wires are one thing on cars that I cannot get my head around (except very basic stuff).
Thanks for the reply tiff_lee.

Lets have a look then.
 
Is this a factory fitted item or aftermarket?  

I'm thinking it's after market now.
I just googled pics of dashes and what I have is not in any pics.

Ah yeah, you will need to get a lot more personal with the car than that.  Probably be easier if you remove the lower part of the dash on the drivers side rather then getting your head up underneath it, you need to find the control unit and trace the wires where it splices into the cars loom.

Sounds like a job for someone else hahaha.

It won't be difficult, easy money for someone!

I've got a mate that will handle this no probs.
Cheers.