Instrument Panel Electrical Issue???
 

Hoping to get some expert input on an issue I can not figure out.

Some Back info;
Jan 2019, I purchased a 2000 Montero Sport XLS 4x4 with 160k miles on it. One thing I noticed when I placed the vehicle in Drive, the "D" on the instrument panel would not light up. All others work. Mid February, I had a viper alarm installed by the authorized dealer where I purchased the alarm. When I drove away, I noticed to my surprise the light on "D" was now working!!!!

Now to my issue, This past weekend, March 9-10, I replaced Spark Plugs & wire set, valve cover gaskets/tube seals and air intake plenum gasket. All was fine after all that work, drove a full tank (approx 280 miles) with no issues. Today, 3/14 I noticed the "D" light is no longer coming on.

Any ideas?

There are tiny bulbs that illuminate the drive letters in the instrument cluster. The bulb might be going out, or the contacts to the board getting bad. You can pull the instrument cluster and replace the bulbs. You might need to get them from the dealer, not sure if they are available online.