re: Speedo - is attached to the transmission near the rear of the housing, secured by a pinch fitting with a 10mm bolt that must be completely removed to detach the cable. It is also secured in the driver's footwell by a clip behind the brake pedal, and should be at least disconnected there before removing the instrument cluster (manual says to disconnect at transmission as well, lest you stretch the cable housing).

Sounds like you found the vacuum line to the transmission modulator, the other black cable should be the speedo one.

re: tach - this I believe is indeed an electronic signal from the engine. Don't know much more than this, unfortunately.