I'm trying to install a DH123D decoder in a Life like P2K PA loco and make the mars light work.
Because the DH123D is a two-function decoder (front headlight, rear headlight), it may not be the best decoder choice for this locomotive.
In practice, some PA's had only one headlight, thus there would be no Mars light.
The DH123D would be an appropriate choice for those locomotives.
Typically, eastern railroads would just have one headlight; think Pennsy and Southern Railway.
Western railroads, because of higher train speeds, often used warning lights such as the Mars Light or Pyle Gyra-Light; think Santa Fe, Southern Pacific, Missouri Pacific and many others.
One of the locomotive's headlights would illuminate the track ahead while the other light would alert non-railroad traffic of the train's speedy approach.
These locomotives would be better served by a "146", "166" or later series decoder.
One of the available functions (say "F1") of these decoders would be assigned and wired to the warning headlight bulb, with the CV's for that function adjusted to create the flashing of the Mars light.
With the PA, this would have been the upper headlight, but there are sure to be variations depending upon the railroad.
Our decoder selector guide lists the DH166D or the DH126, with the choice being based upon the presence or absence of the warning headlight.
Over the years, many model locomotive manufacturers have become much more sensitive to prototype accuracy.
So, it is a rare day when a detail such as the warning headlight would be inaccurately placed.
If your PA only has one headlight, it is because the prototype locomotive only has one headlight.
If, on the other hand, if your locomotive has two headlights, then you will need a decoder that can address more functions.
There also may be the need for internal modifications; please see the related articles for possible guidance.
That said, if you have a single headlight PA and are determined to have a Mars Light, hang the accuracy, then giving CV49 a value of "12" or "22" should give you the Mars light action.
If you run into trouble, you can always reset the decoder by giving CV8 a value of "08", which will reset everything.
Also, since the PA was really a single end locomotive with a small backup light for yard use, it might be possible to rewire the rear headlight to serve as the Mars light, adjusting the CV's accordingly.