germany travel Amsterdam to Munich back to Frankfurt

As you will not return your car in the Netherlands, it can incur steep fees for cross-border rentals.
It would be more convenient and cheaper to take the train from Amsterdam to your first destination in Germany, and rent from there. Within Germany, you usually don't pay surcharges for one-way rentals, at least not with the major well-known companies.

Which region do you mean by "wine country"? Most of west/central and southern Germany is wine country
The Rhine valley, and even more the less touristy Mosel valley could be excellent starting points for your road trip.
Go South from there through Alsace/Lorraine, Black Forest, along Lake Constance, northern ridge of the Alps to Munich.
IF you have enough days planned for such a trip! Which is a major factor for suggesting an itinerary. You have any ideas yet for how long you will be coming?

we will picking up our car in Cologne, driving to Rhein valley below Koblenz then to Mosel. From there we are having difficulty deciding our route and wondering if we have enough time to get to Munich via The Black Forest. Then back up to Frankfurt for departure. We have 8 full days to do This. I think it might be a bit much and wondering what we should skip?

You would have enough time to go to Munich through the Black Forest, you just wouldn't have enough time to enjoy it. Enjoy the Rhein Valley and the Mosel.

That's a very large driving loop. You are talking about driving around 1,000 miles and spending 15 hours in the car. I don't think it's worth the expense or the time for 8 days. You'd want to spend at least 2 days in Munich, I'd think, and a car is more of a liability than an asset there. It seems to me that by picking the Black Forest and Munich you are driving right past a lot of really interesting towns, cities and areas that are much closer to your final destination of Frankfurt. A shortened loop would be more efficient:

Michelstadt: http://www.diekieselsteinchen.de/bilder-michelstadt/michelstadt-header-1.jpg
Bad Wimpfen: http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Bad_wimpfen_blauer_turm.jpg/800px-Bad_wimpfen_blauer_turm.jpg
Schwäbisch Hall: http://goeurope.about.com/library/phot/bl_schwabisch_hall_8.htm
Nördlingen: http://wikitravel.org/upload/de/thumb/0/0f/Noerdlingen_Rathaus.jpg/300px-Noerdlingen_Rathaus.jpg
Weikersheim palace: http://www.main-tauber-kreis.de/media/custom/266_3712_1_k.JPG?1196915403
Rothenburg: http://images.travelpod.com/users/lindakhaas/1.1282254880.the-old-forge-and-city-wall.jpg
Bad Windsheim (open air museum): http://www.stripes.com/military-life/travel/bad-windsheim-time-travel-at-franconian-open-air-museum-1.102354
Würzburg: http://www.sy-tongji.de/2010/6/main_-_wuerzburg/wuerzburg_mainbruecke.jpg
Bamberg: http://eurobuildings.info/wallpapers/germany/bamberg_w003.jpg
Gelnhausen: http://www.malsam-online.de/fotoalbum/deutschland/gelnhausen/gelnhausen_17.jpg

Russ,
Thanks so much for the advice and great sites we will take your advice. Any suggestions for places to spend time in the Rhine river and Mosel valley, even lodging would be helpful.
BE

"places to spend time in the Rhine river and Mosel valley"

Rhine:

North of Koblenz: Linz; very attractive old-world village on the east bank. Remagen (west bank); excellent WW II museum: www.bruecke-remagen.de

Braubach (south of Koblenz, east bank); attractive town, and awesome Marksburg Castle (tour):

www.roadstoruins.com/marksburg.htm - www.marksburg.de - www.bensbauernhof.com/pensionfelsenkellerbraubachrhine.html

(All the rest south of Koblenz on the west bank)
St. Goar; Rheinfels Castle, www.st-goar.de/17-1-.html - have a meal at the castle hotel there - great view from the terrace restaurant "Burgschänke der Landgraf" - photos: www.gourmetclassic.de/gastronomie/Romantik-Hotel-Schloss-Rheinfels.St--Goar.html

Boppard; nice town with lots of places to stay and dine, nice town square with outdoor dining, wine cellars, cool chairlift ride - www.sesselbahn-boppard.de/

Bacharach and Oberwesel: Fine old half-timbered houses and old town walls, some nice cafes and wine spots. Bacharach is a bit too popular with American tourists thanks to travel cult hero Rick Steves. Oberwesel's "Auf Schönburg" castle-hotel is top drawer:
http://hotel-schoenburg.com/

Mosel:

Cochem; the charmingest of charming places. Great cobblestoned, half-timbered old town with old town walls, narrow streets and alleyways, attractive shops and cafes, lots of hotel choices. Take a river cruise to Beilstein, nice chairlift ride there as well. Be sure to visit the castle for the falconry show (dark Mondays); photos: www.falknerei-reichsburg-cochem.de/fotobuch.htm

Trier; a must if you want to see Roman ruins/artifacts, otherwise not a necessity.

Bernkastel: Fantastic half-timbered buildings, doesn't take long to see.

Good source of accommodations options:

http://www.bensbauernhof.com/accommodationsrhinemosel.html

You could indeed spend a week here easily. An acquaintance of mine once spent a month in Cochem and still didn't want to leave. I'd plan on 3-4 days for the area at a minimum.