Anyone knnow the guage of the wires in the loom? The wires that run upto the handlebars switch gear.

Nope.

And I ain't got my service manual to hand, either... Dammitt...

Have you a caliper to hand or something?

First of all you need to work out what type of wire is used in the loom, there are two standards; traditional and modern thin wall. This will effect what wire you need.

Second don't confuse wire diamenter with cross sectional area. wire is identified by the area of the conductor. A wire with a 1mm squared area is too big to fit in a 1mm dia hole. If you measure with calipers you will find the wire diameter.

Most standard wire in the loom has a 1mm squared area, main feeds will be bigger.

Stick to what ever standard is used on that loom. I know the thin wall looks too thin to most people, but its the better option.

i agree with speedshop, go for thin wall cable as it carries a higher load than traditional hence its better,also, in a handlebar application it'll be less bulkier.There's no reason you can't add thin wall wire to a traditional wiring loom

Use good old 'ohms law' to calculate what current (amps) is to be used by the handlebar switchgear and buy that size wire .

a 55w 12 volt headlamp bulb will pull 4.5 amps, a couple of 21w indicator bulbs will pull slightly less, so on the strength of that 11amp wire will suffice or play safe (as it's thin wall) and upgrade to 16.5 amp wire.