The first clue I am seeing is that you are setting a VSS code without moving the car.
Sounds like you have a wire that is shorted to power/ground and feeding other circuits.
Use your DVOM and test the wiring for shorts to power/ground from the VSS to SMEC.
Fault Code 15 Speed Sensor Circuit
With diagnostic read-out tool connected to engine test connector, start engine, then disconnect speed sensor connector.
Place system in Engine Running Test Mode 71.
Connect and disconnect a jumper wire between terminals of speed sensor harness connector.
If display on read-out tool changes, replace speed sensor.
If display on read-out tool does not change, proceed to Test 3 step B.
DR-3B Speed Sensor Circuit to SMEC For Open
With ignition switch in Off position, disconnect speed sensor and SMEC 60-way electrical connectors.
Connect an ohmmeter between terminal No. 48 of 60-way connector and white/orange wire terminal of speed sensor electrical connector.
If ohmmeter indicates continuity, proceed to Test step C.
If ohmmeter does not indicate continuity, check wiring from terminal No. 48 to speed sensor for an open circuit.
DR-3C Speed Sensor Circuit to SMEC For Open
With ignition switch in Off position, disconnect speed sensor and SMEC 60-way electrical connectors.
Connect an ohmmeter between terminal No. 4 of 60-way connector and black wire terminal of speed sensor connector.
If ohmmeter indicates continuity, replace SMEC.
CAUTION: Before replacing the SMEC, check the G7 terminal in cavity No. 48 and the N5 terminal in cavity No. 4 of the 60-way connector to make sure they are not damaged causing a poor connection.
If ohmmeter does not indicate continuity, check wiring from speed sensor for open circuit.