Volvo 240dl station electronic problems, alternator?

Hi,
I've been reading a lot on alternator problems in this group. My
problem is similar to the ones I've read but just a little different
(of course).
Car: Volvo 240 dl station 1986 manual.
Problem: In a week time the car would start less well untill one day it
wouldn't. Couple of guys pushed me and it worked fine driving for 10
min to a mechanic.The mechanic measured a 12.65 V over my battery with
car running. He concluded that the alternator needed replacement.
After replacement the V was still 12,65. Both alternators were tested
outside the car and both the new and the old were good. The battery was
also good although empty. The mechanic put my old alternator in,
charged my battery and my car ran fine for a couple of weeks (also
after not using it for 10 days, so no leak while standing still I
guess). Then it started starting less easy again untill it wouldn't, I
got a push again, drove home (I think the engine almost stopped a
couple of times). The next day I went to my garage were I explained the
story. After checking it out they told me it was the alternator and
maybe also the alternator belt that needed replacement. Thanking them
for nothing I went home with an according to them fully charged battery
(altough they were a bit vague). A couple of days later I went for a
120 mile drive (each way) on the way back my dashboard fuelmeter and
engine-temp. meter slowly went down to almost 0 although fuel and temp
were fine. Also the speedometer only showed the right speed when I took
my foot of the gas (otherwise to low a speed). The overdrive stoped
working. To me it seemed the battery was totally empty and the
alternator was not keeping up. I stopped allong the road. The fuel and
temp went ok again but as I moved on everything stopped working as
described before.
Coming home the car wouldn't start.
The V over the battery with the car off is still just above 12. The
lights on the dash with key in position II work (I presume battery lamp
is alternator lamp).
Their are 3 wires from the alternator: Two red and one blue (thinner
one). I think they're ok but i'm no expert. (wich one is the ground?,
blue?)

A few things I did right before problems started occuring: I pluged in
a missing fuse (the one for back-up lights etc.) and replaced some
others. Since than the speedometer has been shooting up and down at
certain moments. This same day I replaced some speaker wire leading it
behind the dash. Since that day the "lights-warning light" also pops on
when I turn on the head lights. It goes of when I turn on and of my
direction lights. ()
Since I thought I might have pulled some wires doing that I checked the
dash wiring. It looked ok apart from to loose wires of which I don't
know if I need them. 1: a solid blue one. 2: a red and white one. Does
anyone know what they're for. (I noticed there was another solid blue
one that was connected in combination with 2 other wires (black and
white and an other color if I recall correctly).

Hope anyone can help me I'm gonna be roadtripping for 3.5 weeks in 2
weeks, hopefully with this car.

Thanks.
Gilean.