There was opposition to the German reunification in the period between the opening of the border and the actual dissolution of the DDR. A couple of weeks after the wall came down, a petition called "Für unser Land" (For our country) was published by several leading figures in public life, including dissidents. This wasn't really in opposition to the "revolution" itself, but rather against the destruction of all the gains of socialist construction in the DDR.
Für unser Land called for the maintenance of the DDR as "a society with solidarity, where peace and social justice, individual freedom, freedom of movement and the conservation of the environment are guaranteed" and expressed concern about "a sell-out of our material and moral values".
The petition was signed by 200,000 people in the first two weeks, including Egon Krenz (the last SED leader of the DDR) and Lothar de Mazière (a CDU politician), up to 1.17 million by January 1990 (and 12,500 negative responses). Strangely enough, the
introduction to the document in the Federal Archive says about these numbers that:
"At the same time the amount of approvals reflects the situation in the DDR in late 1989. Only 10 to 15% of the population still supported the maintenance of socialism and their own state. The vast majority of the citizens wanted either a reunification or a confederation with the BRD as speedily as possible, and to achieve a marked improvement in living standards that way."
I don't know about the methods by which this petition was propagated, but considering that obviously only a certain percentage of the population will bother to sign a petition or participate in an opinion poll, it seems very unlikely to me that the 10-15% who supported this statement were the only ones, and that the rest of the population must therefore necessarily be against it. However, the CDU, now led by De Mazière, ran on a pro-reunification platform and won the 1990 election in the DDR by a landslide, so popular support must have been stronger than just "old nazis" as Mabool said.
Of course, the "improvement of living standards" that they expected now looks incredibly naive. And this election was also a bit questionable, given the vast material support given by the West German political parties to their new DDR sister parties. Between the clout of the western parties and the already present infrastructure of the PDS, the DDR dissident movements that participated in electoral lists outside of the imported western parties were blown away in the "imported election".