Typically relay terminals would be connected like so:

30: To battery (+) terminal;
85: To ground;
86: To switch (e.g. key-on power in your case);
87: To device (e.g. rad fan in your case).

Lowish voltage at 86 is NBD cos it only needs enough microcurrent to trigger the relay. With power from the battery good at 30, power applied to 86, and a good path to ground from 85, there should be ~12 VDC at 87. Since your 87 reads 0 with the key on, either the relay is bad or the path to ground from 85 is bad.