according to your summary, you have power from the ignition switch to the starter, otherwise the car would NOT turn over. but, you seemingly have no 'run' circuit power.
i have to presume that your radio worked before these symptoms started. is it also safe to presume that you have no headlights when the ignition is on, and no wipers? all of those combined would indicate a bad switch.
checking the circuits with a multi-meter is a really good idea. or, swapping your switch for a known good switch from another running Spider would work very well too. (pays to have two running vehicles in your garage...or a buddy).
NEW replacement switches i understand are not a 100% swap for factory original; and as a result, i have heard of guys having trouble getting them to work. a quality used unit may be the way to go for you. Vick Auto sells a new replacement for $140.00 .. IAP sells one for $100.00. i have a used one i will sell you for substantially less; it is from a '78, which should be 100% compatible with what you have now. if you are interested, send me an email -- but please use the email posted in my signature, not that funny icon thing.