Is that a unit including the brushes?

Set MM at lowest Ohm range, short both leads (red+black) - what do you read?
Probably 0.something - whatever it shows, that's your zero value.

Now, with all the car's wiring in place, put the MM black lead on the battery's
-(minus), red lead on the alternator ground connection - what do you read?
Should be same or only slightly higher than previous 'zero' reading.

Try another battery or get yours tested.

First thing to check is the charging lamp in the dash - if that lamp has
blown (or is the wrong wattage) the alternator won't be able to charge the
battery properly or at all. I'm not exactly sure where in the circuit the
lamp sits but there are notes in various service manuals to make sure it's
the right wattage and check that it's not blown when looking at problems
relating to the the battery not charging and therefore not being able to
provide 'secondary regulation' for the output of the alternator.

You should get between 13.7 and 14.5 volts at the battery terminals with the
engine running and the voltage regulator on the alternator producing output.
If you get 13.3 volts there's definitely something wrong, and when you
switch off the engine with a voltmeter connected across the battery
terminals you should see little or no decay in the voltage below about 12.5
volts after the key is out (which turns off just about everything
electrical).

If you disable the fuel pump and crank the engine what happens? If you see
the voltmeter reading die back to 10 volts or less and the engine either
won't crank or tries but gives up, that's a sure sign that the battery
hasn't been charged enough. You might want to first off try a different
battery just to confirm that's the existing battery isn't the problem, and
if it's not, work through the charging circuit checking each component and
wiring between components as much as you can.

I had a good brand battery in my 1983 900S for just over a year after being
bought brand new and it developed a collapsed cell which meant that no
matter how much charge it had pushed in, it 'leaked' charge internally and
the voltage would collapse quickly. I also replaced the voltage regulator
(bought a new one from one of the Ebay sellers marketting them) and that now
gives a nice 14.5 volts on the terminals of the replacement battery when the
engine is going.

Regards,

Craig.