Can’t figure out how to pull it out. Feels like the speedometer cable is holding it in, but I can’t reach behind it to unhook it.
Yes the harness plug and the speedo cable need to be unplugged. Some people drop the steering column, others remove the drivers seat so they can lay flat and reach up there to access. Because it’s such a fun job I suggest you replace all the bulbs while you have it out.
Sometimes, it’s just easier to take the top off the dash, and reach down to remove the cable. If your car doesn’t have factory a/c, it should be easier to access from the top.
On my 73 with a/c I couldn’t get to the harness or the speedo from the top Just my experience
Back then I didn’t drop the column nor remove the seat, but I was a younger more flexible gent back then 
I have removed them both ways from the top and the bottom. My experience is that it is easier from the top even with an A/C car. The A/C duct can be wiggled out and then reach over the top, you should be able the get your hand on the dash connector and speedo cable. Doing it from the bottom, I had a hard time finding a path through the under dash maze to get my hand into to reach the connector
66-72RAB is right. It’s a POS trying to get at them from the bottom. I’ve had my '72 xr7 speedo cable and inst. connector out from the top, and mine has A/C. Some A/C car’s tubes are weather-worn and extremely brittle if left in the sun a lot, and then you just plain break it trying to remove it. If you suspect your duct is toast, they are available, but maybe pricey. Mine was entirely toast, so I bit the bullet and bought good used ones from Don over at WCCC. You might get lucky at ebay. But…that’s assuming the worst case scenario. Good luck, and keep the faith.
I have had mine out a number of times. I do it from the top. Two screws and it pops off. Just make sure you don’t break the plastic housing of the speedometer cable or you’ll be buying a new cable. Good luck