GT Car Parts might be a good place to check for the cables. They make their own for the Jalpa. I bought one for my choke on the Jalpa and it was a perfect match. They also might have a lead on a heater core. Call Bill or Dave at:
(623) 780-2200

Also, Eurospares is parting out a Jalpa, maybe they still have the heater core and valve? http://www.eurospares.co.uk/breaking2.asp?page=54

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Mike
Jalpa #12071

Aerodynamics are for people who can't build engines -- Enzo Ferrari