Ok, here’s what I remember…
It’s a movie from the 70’s or 80’s.
There’s a scene where two women drown a man in a bathtub.
His body is put in a trunk.
The trunk in en route somewhere in a truck or something and begins to leak.
That’s all I remember. Does this ring a bell with anyone? It’s been making me nuts! I remember watching it with my grandmother when I was a child and being very frightened. Someone please tell me if this movie sounds familiar.
Sounds a lot like “Les Diaboliques” or the terrible remake of it.
The Internet Movie Database only has one entry for “drowning-in-bathtub” in their Keyword Power Search.
http://us.imdb.com/Title?0068708
Home for the Holidays, 1972, a.k.a. Deadly Desire.
However, the minute I read the OP I said, “Sam Waterston, where they dump him in the pool”.
Reflections of Murder, 1974.
http://us.imdb.com/Title?0072070
Here’s the original, Les Diaboliques, 1955. I doubt whether that would be what you saw in the 1970s or 80s, unless you and Grandma were at an art film house. 
Perhaps I was an an arthouse film festival with my grandmother…
Okay, maybe not.