Get yourself a copy of the Gandini route book (French but easy to follow with GPS co-ord's). Loads of routes in remote places. ISBN 2-913412-06-08.
Route suggestion -
Tarifa (near Algeciras) to Tangier. Fast ferry (45 minutes) same price as lumbering Algeciras ship but no queues and much faster. Customs in Maroc is simple enough. Change money and buy car insurance 10 yards from the customs exit.
Then look at Asilah 1 hr south of Tangier, good place for first night. Get Lonely Planet Morocco guide for accomodation possibilities.
Head to Casablanca on toll motorway and then turn towards Marakkesh but bypass completely Marakkesh using country roads which you can do later when more confident -now towards Agadir but inland via the spectacular Imi-n-Tanoute route, not the coast road. Side route to Tarfaya/Laayoune if time is available - Big dunes/Lagoons/Birdlife/Shipwrecks and a taste of further south)
Otherwise explore the mountainous but agricultural area around Tafraoute and head to Tata for the start of desert pistes. They are all in the Gandini book or I can send you the routes in TTQV form or text emails. Basically there are about 1000 miles of absorbing pistes between Tata and Bou Denib. Really you shouod travel 2 up on some of these pistes, but it's not essential if you have a sound vehicle.
You need a GPS to view the routes, a TTQV mapping programme, a Michelin 742 Morocco map for planning, IGN maps of Morocco on CD for more precise planning, Lonely Planet, a good car and 5 tough tyres in good nick, plus water reserves etc. Of course you may get away with less gear and trust to luck.
Hope that helps,