That symptom is because of huge draw of power by your glow plug system. It
means your alternator is failing. So I'd put in a new regulator on it to see
if you can still keep your alternator. Two screws on back of the
alternator... very easy job.
Alternator must provide at least 13.5V at idle to charge battery...
Otherwise, it will just drain the battery further.
How old is your battery? Maybe you should have the charging system checked
by Autozone... free of charge.