Can I drive my Spanish REG 1989 Ford Escort in the Netherlands?

As long as you still live in Spain, you should be allowed to drive a car registered in Spain in the Netherlands without any special formality. Note that two cities in the Netherlands, Rotterdam and Utrecht, have special rules banning cars like yours from the city centre (cf. official information regarding Rotterdam, Utrecht only bans old cars with a diesel engine). Amsterdam also has restrictions but only for lorries and delivery vehicles.

Theoretically, if you gave up your place in Spain and moved with your girlfriend with an eye towards living permanently in the Netherlands, you might be required to register as a resident and that could have a number of consequences (taxes, health insurance, car registration and insurance, driving license, etc., etc., etc.) But as long as you don't need to start working or open a bank account and only stay for a few weeks during the summer, I wouldn't worry too much about it.

In practice, I was even able to get a resident's parking permit for a car with foreign number plates for almost a year (close to the maximum time allowed before registering it in the Netherlands at the time).