I built this one
http://www.moparts.org/Tech/Archive/elec/11.html

it works and isn't that expensive.

there is one made by a company for around 25 bucks.
I think this one cost me 4 bucks in parts.

I am not sure it would survive 18 volts either though.

also be sure the gauges are dead. tie 2 AA batteries together serially and apply them to the gauges.
if they move (and it takes a second before they move) they will only go to the middle and you will know they are ok.