Climate Control

I have and 99 E300 TD. Passenger side is blowing cold, driver side is blowing netrual temp. Where should I start?

Thanks.

Scott

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1999 E300 DT 178k
1960 Ford Thunderbird Convertible 82k
2000 BMW R1200 CE 12k
2000 Nissan Xterra SE 145k
1994 Go Ped 1,000,000 miles