The existing deck (4x50) runs the in boat speakers.  That is the only part of the system that works right.

The 1500 is only (at the moment) set up to be the subwoofer amp.  The old preamp connections to the previous sub amp (p600-2 - using only one channel as Malibu had it wired) is now plugged into the 1500 sub channel input.  The sub channel output goes to the brand new sub, which is 4 ohms.  New amp, new sub.  New sub responds to independent signal.  Amp lights up normal.

As luck would have it, one of the tower speakers started cutting out last Friday.  Today neither work.  (Powered by a different amp altogether).  Do these boats ever have everything work right all at the same time?  Lol

2 hours ago, Nick55 said:

It sounds like you may have a bad alternator, BUT, it could be a loose belt, or possibly a loose (big, like an engine/battery ground, positive battery cable to the engine starter/alternator) connection too.  
 

If I’m way elementary here, I’m sorry….. I’m impressed with the dash retrofit you’re doing but I don’t know your mechanical knowledge.  No disrespect meant here if I am breaking this down too far.  

First, let’s understand the components and function of an alternator.  1990’s saw some changes to them but essentially, they all have a regulator that co trolls charging.  

Regulator

The voltage regulator controls the amount of power distributed from the alternator to the battery in order to control the charging process. Regulators are designed with different functions and work depending on their specification.

Rectifier

The rectifier is used to convert current from alternating current (AC) to direct current (DC) during the charging process.

Rotor

The rotor is the spinning mass inside the alternator that rotates via the pulley and drive belt system. The rotor acts as a spinning electromagnet.

Stator

The stator consists of several coils of wire wound through an iron ring. The stator sits outside the rotor, when a magnetic field is created the electrical current is made.

Drive End Bearing

The bearings are designed to support the rotation of the rotor shaft.

Pulley

The pulley is connected to the rotor shaft and the drive belt system. Rotation created by the engine the drive belt system turns the pulley beginning the charging process.

(Above quoted from here) https://www.autoelectro.co.uk/alternators

Now depending on the alternator type (GM used some one wire and some with an external regulator in that era) you test them a bit differently. Almost no alternators work well at idle, so my first question is:

What happens when you raise engine RPMs and you measure voltage at the battery? What about if you measure at the large terminal stud (not the cable, actually at the stud) on the alternator (+) to the engine block (-)? You should read close to the same (within a tenth of a volt). If the alternator reads higher, you need to check your cable connections to the battery.  There is also an exciter wire that tells the regulator what battery voltage is (I don’t have that diagram, and that usually feeds the gauge as well).   
 

this has some decent reading in it too. 

 https://www.jalopyjournal.com/forum/threads/gm-one-wire-alternator-problem.199627/

No disrespect taken!  Thank you for offering some help, good points.  Here's what I know:

• No change in voltage when I revved it at the battery nor at the alternator.  That said, I was on a garden hose, so I really would not go over 1,000 RPM so as not to starve my impeller of water.  But I duplicated my on-the-lake-experience from last summer: at idle, cruising around the lake at night time, if I was strictly at idle (600 RPM) I could tell the lights were dim and I was not charging.  If I bumped the throttle a bit to 1,000 RPM it would brighten up.  I had no lights or stereo on while doing the test today, and didn't measure any change in voltage at the alternator nor the battery
• Stud on the alternator and engine block were my + and - locations to pull the measurement from.  Battery voltage and alternator voltage were both exactly 12.25
• The belt is tight, replaced in 2013, 200 hours on it
• Additional info:
  • The alternator appears to be the original Mando 817119.  I have owned the boat since 2008 and I have not replaced it, it has about 860 hours on it.
  • 
  •  

The boat is 2 hours away. I’ll give it another try this weekend. As I recall, it would not move off the SERVICE REQUIRED screen no matter what button I pressed.

Since Ford and GM engines from Indmar, PCM/Crusader, and Malibu were all used in 2019 it can get confusing on which side is bank 1.  Bank 1 is the side that has cylinder #1.  Best of luck.

Use the up/down arrow buttons to go to the Setup screen, press E.  Use arrow buttons to highlight Service, press E, then use arrow buttons to highlight Fault Codes, and press E.  Note any code numbers.  Press red button to exit Setup.