Help needed on electrical issues (i hope)

[zebedee]

Ive got a 90H 8v and have recently had a full engine rebuild. The engine is sweet and is a pleasure to drive with no problems, however, it is suffering electronically-wise.
Its rev counter works when it wants, sometimes not at all.
Its temp gage light is flashing, especially at idle and pull-up.
Occasionally my general cabin instruments i.e wipers, heater don't work and then just kick in 5 miles down the road.
On start up a strange plasticy-tapping sound can be heard just behind the heater control/ Cd player.
I know naff-all about electrics so any help is much obliged. 

[sparkplug]

have a look at the back of the fuse box (above the drivers pedals), check for loose cables there is also a common earth block just above the fuse panel , could be loose or dirty connections

[gazareth]

when my wipers/heater and lights fails it turned out to be the ignition barrel plastic switch. worth a look fella.