Volumetric Lighting Help

I am trying to make a custom solution for volumetric lighting. I have a camera set up on a spotlight which renders a depth texture. I then pass that depth texture and some other information about the light to an image effect shadow to actually draw the volumetric light. However, the light does not seem to be working properly. I am not using proper code to account for the spotlight's cookie and angle of view, but the light shafts are not working properly either.

Here's what happening (the volumetric fog is rendered, but light shafts are not):

Here is the code for my shader:

 Shader "Hidden/VolumetricLightRenderer"
 {
     Properties
     {
         _MainTex ("Texture", 2D) = "white" {}
         _Shadowmap("Shadowmap", 2D) = "white" {}
         _LightColor("Light Color", Color) = (1,1,1,1)
         _ShadowmapPos("Shadowmap Pos", Vector) = (0,0,0)
         _ShadowmapDir("Shadowmap Dir", Vector) = (0,0,1)
         _ShadowmapNear("Shadowmap Near", Float) = 0.1
         _ShadowmapFar("Shadowmap Far", Float) = 50
     }
     SubShader
     {
         // No culling or depth
         Cull Off ZWrite Off ZTest Always
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             
             #include "UnityCG.cginc"
             #define SAMPLES 12
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
             };
 
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 float4 vertex : SV_POSITION;
                 float3 viewDir : TEXCOORD1;
             };
 
             v2f vert (appdata v)
             {
                 v2f o;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.uv = v.uv;
                 o.viewDir = normalize(WorldSpaceViewDir(v.vertex));
                 return o;
             }
             
             sampler2D _MainTex;
             sampler2D _Shadowmap;
             fixed4 _LightColor;
 
             fixed4 _ShadowmapPos;
             fixed4 _ShadowmapDir;
             fixed  _ShadowmapNear;
             fixed  _ShadowmapFar;
 
             float fract(float x) {
 
                 return x - floor(x);
 
             }
 
             // Source: http://stackoverflow.com/questions/4200224/random-noise-functions-for-glsl
             float rand(half2 co) {
     
                 return fract(sin(dot(co.xy, half2(12.9898,78.233))) * 43758.5453);
 
             }
 
             fixed3 calcVolumetric(fixed3 ori, fixed3 dir, float dist) {
 
                 half3 col = half3(0.,0.,0.);
 
                 float is  = dist / 50.;
                 float vrs = dist / float(SAMPLES - 1);
                 float rs  = rand(_ScreenParams.xy) + vrs;
 
                 half3 samplePos = ori + dir * rs;
 
                 for(int i = 0; i < SAMPLES; i++) {
 
                     half3 sampleDir = _ShadowmapDir;
                     float depthSample = Linear01Depth(SAMPLE_DEPTH_TEXTURE(_Shadowmap, samplePos));
                     float fragmentDepth = _ShadowmapNear + (distance(_ShadowmapPos,samplePos) / _ShadowmapFar);
 
                     if(depthSample > fragmentDepth) {
 
                         col += _LightColor * is;
 
                     }
 
                     samplePos += dir * vrs;
 
                 }
 
                 return col;
 
             }
 
             fixed4 frag (v2f i) : SV_Target
             {
                  fixed4 col = tex2D(_MainTex, i.uv);
                 fixed4 vol = fixed4(calcVolumetric(_WorldSpaceCameraPos, i.viewDir, 3.), 1.);
 
                 col -= vol * .5;
                 col += vol;
                 return col;
             }
             ENDCG
         }
     }
 }

Anybody know what my issue is? Help would be greatly appreciated. My only guess is that I am not finding the depth of the sample properly, but I'm not sure. I'm pretty nooby with Shaderlab and I have never touched Cg/HLSL outside of Unity in my life :P

Thanks in advance!