Add (colon-separated) `<directories>' to the front of the default path used to follow `\input' commands (just `.' at the moment). Only give one `-I' option; I cannot promise what will happen if you give more than one.

Slight magic: the `.' in the path means: first, look in the directory where the file doing the `\input''ing is; if not there, look in the current working directory. (This is usually what you want.)