I know, but I've been told that Spain has the most underused installed capacity.

Does Spain export a lot of electricity? It could produce a lot of solar power. Even some in the winter, a lot in the summer when it's needed for air conditioning. One can think of it this way, if a solar panel is manufactured in or imported to EU, maybe it should be installed in the more sunny regions for maximum benefit.

Spain is also a wind power giant.

As comparison, Germany is not a great location for either wind or solar power.

I heard that France refused several projects to connect the iberian system to central europe properly, not only electricity but also gas.

It isn't that spanish companies can get much help from the government's diplomacy (long topic), so someone else interested in that connection would have to partner with such companies, or a foreign diplomatic body go and do the legwork for the spanish companies, which wouldn't be the first time.

Germany has lots of good locations for wind, the problem is that anyone can block them, because "they don't like how they look, are noisy, too close to their home,..." and plety of other reasons.

Thus Germany could be like we have it back home in the Iberian Penisula, but no, many people are keeping the courts busy instead.