Facilitative organic cation transporter that mediates the transport of carnitine as well as the polyamine spermidine (PubMed:12089149, PubMed:20037140). Mediates the partially Na(+)- dependent bidirectional transport of carnitine (PubMed:12089149). May mediate L-carnitine secretion from testis epididymal epithelium into the lumen which is involved in the maturation of spermatozoa (PubMed:12089149). {ECO:0000269|PubMed:12089149, ECO:0000269|PubMed:20037140}. This is the function of SLC22A16 (solute carrier family 22 member 16, ENSG00000004809).