RST Digital Engine Monitor-ATTN Jim Weir

I'm attempting to build the RST Digital Engine Monitor (Kitplanes Feb.
2002). I could not find the LCD panel meter specified in the plans or
even the substitute mentioned in the addendum (http://www.rst-engr.com/
kitplanes/KP0202/KP0202.htm) so I tried to substitute one I found at
All Electronics (PM_200). I can get the circuit to work ok except I
need to power the panel meter with a separate 9 v. battery. Whenever I
try and hook it into the common dc circuit it goes into error mode.
Apparently, it needs an isolated power supply. Do you have any idea if
there is a way around this? It would be such a pain to have to
periodically change batteries behind the dash, not to mention it would
undoubtedly go out at the most in opportune moment.

Thanks,
Roger