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You didn't give any details on what car your talking about but in general terms a blinking "D" light indicates that there is a problem with the transmission. This may be something mechanical such as a shift solenoid or possibly some electrical such as a transmission speed sensor. This may also indicate that the transmission fluid may be low as well.
The guys over here on the T100 section have tacticle the issue themselves. I have no experience working with A/T's.
Type of thing that the correct diagonois is gonna be the trick to keepin the expense down. Is or does your CEL come on.The ecm will store pending codes too so even if you haven't seen the CEL I'd start by getting it scanned.
Local parts stores usually will do it for free or you can buy a bluetooth OBD2 Scanner that will display the info on a smart phone for about $25 bucks. Nice to have even if someone else is gonna fix it. Keeps em honest if you have an idea of the problem ahead of time.
Go on the Venza Forum and ask the other Venza owners about the issues, wouldn't surprise me if someone over there has!