Why did my car insurance rate go up after removing a vehicle from the policy?

Your car insurance rate went up after removing a vehicle from the policy most likely because you weren't given a multi car discount anymore.

Companies usually offer a multi-car discount that lowers premiums, and when you go down to one car that discount is removed.

Its impossible to say for certain not being your insurance agent and without knowing more details.

But, likely what happened is that you are a One Person Two Auto Insured.   The old Jetta may have been listed as your commute car and the F150 listed as your pleasure car.  When you removed the Jetta the F150 naturally became your commute rig.   The Ford F150 is worth more than the 98 jetta and may (or may not) be considered a more risk prone car.

This is just one possible scenario.