Ian, John V and I last year had a similar issue with his Alfa before OWR last year as well. Bryn Epp suggested something that has worked ever since, with John's car anyways. You need to wrap the power lead to the odo around a ferrite core. Most electronics places should have what you're looking for. It looks like a little cube, with a hole through the middle, that splits down the middle and will open up. You wrap your power lead around the outside and through the middle a few times, then close it back up. It has the effect of dampening out any spikes in the power.

I recommend twisting the power leads to your radio before you route the harness to the radio.  Otherwise the two unshielded wires act as an open magnetic loop (e.g. antenna) and will pick up loads of crud under the hood.  This is good advice.  Try to route the power leads away from high voltage sources if possible, like ignition or coil wires.  Twist them well, it will take a little more wire, but worth it to knock down this magnetic coupling (the magnetic part of the radiated electromagentics).

Ferrite cores are inductive isolation or filtering (magnetic related), and capacitors are umm, capacitive filtering (charge related).  But, I've haven't dabbled in electrics world for a long time and can't say much more than that.  

Btw, if you have crappy cables under the hood for stuff like your ignition (e.g. bad coil wire) it will radiate pulses into your odo signals, and your odo will be off.  I learned this the hard way when re-measuring a rally section once.  So, try to keep all your OTHER wires and connections in good order or it'll turn your lovely Alpha into an overpriced  random number generator!

You have a ham ticket?  Cool.  Oi, turn off the radio when rallying.  You might hear something. .. . I think I'll have to start planting false info into the Lanark comms!  Too many clever arse hams on the  teams nowadays.

Btw, Ian, those prizes offered at OWR sure sounded great (and not cheap).  Thanks for supporting the grassroots!!

Sounds like I need to write an article on cleaning up power lines in cars.

^%$# I opened my mounth!
Now, were's that word processer

Looking at all the responses It looks like all the types of noise fixes have been used here.  Now to explain them in that article.