87 (not 87a) is the power from the relay to the fan (by the diagram above) so you can hook your LED + to that.
Wiring the (+) leg of the LED into the fan relay #87 will ensure the LED is indicating the fan is ON since the LED will be only drawing current when the fan is ON. That's what you want to see on the dash when it's lit.
All you're doing is taking an LED (preferably the number listed above from Rat Shack since it's made for 12VDC and doesn't need a resistor), soldering (preferably) a wire to the short leg (-) and a wire to the long (+) leg (LEDs have one leg longer than the other). Connect the wire from the short leg of the LED to ground (ground behind the dash, frame, whatever). Connect the wire soldered to the long leg of the LED to #87. Obviously you'll need a few feet of wire to go from the fan's relay to the location on the dash where you'll install the LED. The wire can be pretty small since it's not carrying any current. Make SURE the wire from the fan relay to the LED is wrapped, covered, heatshrunk, whatever, no bare wire or connectors touching anything. LEDs are cheap so buy more than one just in case you cook one.