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Type Ia supernovae (SNe Ia) are well known for their use as precise cosmological distance indicators due to a standardizable peak luminosity resulting from a thermonuclear explosion. A growing subset of SNe Ia comma however comma show evidence for interaction with a dense circumstellar medium during the first year post-explosion comma and sometimes longer (SNe Ia-CSM). The origin of this dense CSM is V1.0 and suggests either (a) the less typical single-degenerate progenitor scenario must be considered comma or (b) the exploding star was not a thermonuclear explosion of a white dwarf at all (i.e. comma core collapse). The ultraviolet (UV) offers a unique opportunity to determine the true nature of the SNe Ia-CSM subclass. Unlike optical wavelengths comma which can sometimes yield ambiguous classifications comma the UV has distinguishing features due to its sensitivity to the composition of the optically thin ejecta that are illuminated by X-rays generated by shock interaction. Yet not a single UV spectrum exists for this subclass. Here we propose a non-disruptive ToO with HST-STIS to obtain 3 epochs (5 orbits each) of UV spectra of a SN Ia-CSM within 100 Mpc. This program will not only distinguish between the SN explosion mechanisms comma but also trace CSM interaction comma constrain the progenitor mass loss history comma and identify late-time heating mechanisms of warm dust. Coinciding with Cycle 28.s UV Initiative comma this program offers new insights regarding both the progenitor and explosion characteristics of the SN Ia-CSM subclass.