I found THIS http://www.bas.uk.net/docs/GPSNAV5.PDF in the Cambridge
manual:

Model 20/25 power control is more complex. Un-switched power (using the
Red wire) means the GPS-NAV is turned on as soon as power is applied.
The GPS-NAV disptay ON switch is not used. You must use this connection
if the GPS-NAV is to be used without its LCD display and without
connection to a Cambridge L-NAV.

Switched power (using the Green wire) means the Model 20/25 GPS-NAV
responds to the ON switch. Power may also be slaved to a Cambridge
L-NAV. When the L-NAV is turned on, the GPS-NAV turns on automatically.
When the L-NAV is turned off, the GPS-NAV waits 30 seconds and then
turns itself off.

Have a look at the LINK http://www.bas.uk.net/docs/GPSNAV5.PDF for more.

Good Luck!

On 5/22/2017 7:09 PM, wrote:
Anybody know how to wire the model-20, so that the "on" button works? Red and black will turn it on, but the "on" switch doesn't work.
JJ

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Dan, 5J