Here's the one I use on a 65 lb Lab.  (Probably about the same length of
dog but with less girth.)  The "restraint strap" clips into the seat
belt.  <
http://www.champk-9.com/autosafetygear.htmhttp://www.champk-9.com/autosafetygear.htm

Last Lab did OK on long trips.  Current Lab stands for the first couple
of hours of any long trip.

Suggest Google for something with wider web straps for your Rotweiller.
 Try hunterk9.com.

My dog weighs about 90 pounds, too, and I'll have to admit, she teeters on
the edge.  She does lay down a lot, though, so that helps.  God it is funny
though because she comes so close to the top when she's sitting up straight.

Anyway, Momo is a type of speaker that Polk makes.  I realize there is also
a brand of equipment (I don't know what) named Momo, so I should be careful.
But, Polk Momo's are one of their top speakers.  The ones I have are 41/2
inch with the tweeter mounted independently in the middle of each one.  The
tweeter can be aimed.  I don't have the spec sheet with me, but the range is
pretty awesome.  The bridge is external.  It's a box about 2 x 4 inches
which contains the electronics for splitting the signal range to the tweeter
and the midrange speaker (2-way).  I built simple boxes for the 6 x 9s and
compression fit them in the rear where I am using the hollow body right at
the sides of the rear bumper to create the echo chamber.  I coated those
with expanding foam.  The sub is in the jack compartment, and it is covered
with a grill I made to keep it from getting damaged.  The inside of that box
is again built with expanding foam for the echo chamber.  I have the little
200 watt amp tucked along side of it.

You know, I just realized I'm yapping along not paying attention to the fact
that we have two different model cars.  I have a C3.  1977.  I don't know if
any of this matters now.

I use the equalizer in the Pioneer head unit to cover all the ranges of
sound to all the components.  In my other car, I have an amp that has RCA
out for the front and rear speakers, and also has an RCA out for the sub.  I
get better balance of tone from that, but then again, I'm trying to fill up
a van instead of a Vette.

What I really like is that I can go for the  low range sounds without
bothering anybody outside the car.  And I can talk to a passenger without
yelling.  To me, that's always been one of the signs of a good sound sytem.

I've found that with some ingenuity, I can fit just about anything into
anyplace.  Backwards as usual, I just make the best deal I can on the
highest end speakers I can find, and make it all go from there.