In the Find field just type <spacebar>=$ (<spacebar> means hit the bar to type a space). The $ at the end means the end of a paragraph, so it won't find any instance anywere else in a paragraph, which is the only reason you need GREP to do this instead of a plain text search.
Set the paragraph style you need in the Change Format.
This only works if it actually IS the end of a paragraph, however. If you have multiple lines in the same paragraph that end in =, or more text in the paragraph after the =, you need to show us the structure. Keep in mind that paragraphs can have only one paragraph style, applied to the entire paragraph. If you need multiple styles within the same paragraph you must use character styles, which can be applied as nested styles, line styles, or GREP styles, or manually, depending on the text structure you have to work with.
If i understand you, Try this grep:
Salah, your expression finds all text from the start of a paragraph up to the last instance of <space>= in the paragraph, if <space>= exists, but does not require that the paragraph end in <space>=, which it seems to me is what the OP is looking for.
Ah, yes! It was the character style that was messing me up!!! I'll do a find/replace for anything with an incorrect character style AND the correct paragraph style applied, and remove the character style so it will quit overriding the paragraph style. THANK YOU!!! That just saved me hours of work.