I agree with Mike on finding the source by starting it for a moment.
You'll end up with cooler fluid if the external cooler is in front of everything else (radiator, AC condenser, etc.) and is last in the fluid path before return to transmission. Depending on how far North or South in the country you are and how tired your transmission is could decide which way you want to go on this. Some marginal transmissions do funny things when too cold so put the external cooler first and radiator cooler second or skip the external altogether. Some don't care about being cool so run the external cooler second.
Personally, I've almost never had a transmission become sad about being too cold, even in below zero. The exception being one time on my 1981 Bonneville'* GM Metric 250C (never-touched including fliter change with 149,000 miles on it) when it was 2 minutes running after being ice cold for a week. I gave it a pass on that. And later when my parents gave it to me I dropped the pan and found out how plugged up things were.
Others may have different experiences.
Last edited by CathedralCub; 04-06-2016 at 03:17 PM.
Reason: Added "(" and ")" to enhance clarity