Stratus includes a specially tuned, high-performance internal antenna that will get good reception in many aircraft, so most pilots will not require a remote or external external antenna and can thus operate completely "wire-free".
Some aircraft, however, may require a remote antenna due to signal attenuation caused by the aircraft. The remote antenna can also allow you to position the Stratus in an out of the way location in the aircraft while still allowing for good ADS-B and GPS reception. Sporty's sells remote ADS-B and GPS antennas for the Stratus 1S, 2, and 2S (Stratus 1 can only use a remote GPS antenna.)
If you have an unused DME antenna on the belly of your aircraft, your avionics shop may be able to repurpose that to serve as an external ADS-B antenna for the Stratus. Or they could install a bottom-mounted TED type antenna: http://www.aircraftspruce.com/catalog/avpages/ted_transponder.php.