In the following sentence:
What is the grammatical reason for 探｛さが｝す to be used in て-form before ん?
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誰探してんの is a contracted form of 誰探してるの, which in turn is a contracted form of 誰(を)探しているの ("Who are you looking for?", notice the progressive form). More generally:
/r/ + vowelbefore a /n/ consonant can turn into ん in casual speech. (Do not confuse this ん as explanatory-の.)
EDIT: By extension, るんだ can contract to んだ (informal and slangy).
According to the site below, it is the dialect of Osaka.